Electrical Services: Installing Lighting & L1/L2 Feeds

[sophiecentaur]

Yes i have realized to my detriment that this type of question is not suited to these forums!

Likewise, technical and Physics questions would not necessarily get useful replies on an 'electricians' forum.
I thought the regs were clear on the Original question regarding the need for double pole isolation. Isn't that the main point, when you get down to it?

 

[brenfox]

There is no physical barrier inside the switch to separate the two line feeds. It is 415v line to line. This is where the theory and the practical fuse together, which happens frequently in my line of work. The 2 feeds are working independently from each other on a separate circuit breaker servicing lights operating at a voltage of 230volts. So theoretically speaking all is well. But in a practical situation there is a "potential" of 415v inside the 230v rated switch! Maybe i am splitting hairs on this one!

 

[brenfox]

Double pole isolation is not a requirement on a 230v lighting circuit. The neutral does not need to be switched. So long as you run a separate neutral with every line feed you will not receive a neutral shock.

 

[sophiecentaur]

Double pole isolation is not a requirement on a 230v lighting circuit. The neutral does not need to be switched. So long as you run a separate neutral with every line feed you will not receive a neutral shock.

OK, so what type of switch would be used? A double pole? In UK, that would be a LN switch, for feeding a Power circuit.
I know you can buy all sorts of switches but this one would be pretty non- standard in the UK for a mains installation.

 

[krater]

Although it may seem obvious to you, it must be considered that folks elsewhere might not inherently understand terms like "6 way grid switch" or find it implicit that 230v circuits are line to neutral loads. It's not neccessarily your fault, but maybe it will help put in perspective some of the views of a lack of knowledge on what you're doing.

I'm unsure of where you are installing or whether the NEC is a statute or standard, let alone which revision (updated every 3 years). Jim's post is right on when it comes to installs covered in NEC 404, but this deals with voltages between terminals and although I'm probably unfamiliar with the equipment in question it sounds like you're concerned with too great a potential between lines within the switch, not between terminals of a separate swtich?

From my experience (maybe comparable to yours, only in USA), most devices, conductors, boxes and other general materials come in either a 50v class for low voltage or power limited systems which never see mains power, or a 600v class, which is used for normal building wiring even on systems such as 120v line to neutral. Rarely you will run into conductors with insulation ratings of 300v, I'm sure I've seen it (fixture wires?) but it is uncommon. Make no mistake, using a 120v rated switch on a 277v or 347v circuit will swiftly end its utility value as a means for controlling a circuit, however connecting 480v line to line across a residential grade switch is not going to result in, say, energizing every other switch in the box as the excessive voltage "bleeds" out of the misapplied switch.

In doing some research I have found in standard UL 20, which coverse general use snap switches, that under section 5.9 for dialectric voltage withstand testing a switch rated at 120vac must be insulated to withstand a megger test of 1500v between live parts as well as dead parts without insulation breakdown. For 251-600v, the test voltage can be in excess of 2500v.

Does this address your question? I have done my best to understand the application, I doubt that your engineer lacks any experience in the field, ie this isn't his first project. Not that mistakes don't happen, and as for the comment above about how field electricians should not be responsible for project design from square one, well, it is my opinion that between the NEC and fairly rigorous experience a well qualified electrician should be at least as capable of designing a functional and efficient electrical installation from the utility transformer right down to the lamps as any degree-holding engineer. Sadly, electrical engineers are too often painfully ignorant of things like building codes, practical uses and applications, and other aspects of physically constructing a building that electricians must be proficient with on a day-to-day basis. And they ususally don't like it when an electrician demonstrates this either, contrary to some of the sentiments above.

 

[Averagesupernova]

In the USA the neutral is NEVER, EVER, under any circumstance interrupted. Switching the neutral is prohibited. Sophie, I think this is not the case in the UK.

 

[krater]

In the USA the neutral is NEVER, EVER, under any circumstance interrupted. Switching the neutral is prohibited. Sophie, I think this is not the case in the UK.

Asn, you're very close to right about that. However, if you've ever dealt with a cabinet heater thermostat, a 120v motor starter coil in which the overload relay is typically located after the coil (the neutral side), or a four pole break transfer switch, you will find that on some occasions the (utility or system) neutral does in fact get broken. It's not common practice, and as I said as a general rule you're right, but you need to be careful if you're going to invoke NEVER, EVER in the electrical industry.

 

[jim hardy]

it is my opinion that between the NEC and fairly rigorous experience a well qualified electrician should be at least as capable of designing a functional and efficient electrical installation from the utility transformer right down to the lamps as any degree-holding engineer. Sadly, electrical engineers are too often painfully ignorant of things like building codes, practical uses and applications, and other aspects of physically constructing a building that electricians must be proficient with on a day-to-day basis.

nice post, krater.

And they ususally don't like it when an electrician demonstrates this either, contrary to some of the sentiments above.

It's a trait of youth . I've been down that road myself.
We all get graduate degrees from the School of Hard Knocks .
That's what codes are - compilations of lessons learned the hard way.
Observe that National Electrical Code is published not by the academic IEEE, but by the National Fire Prevention Association.

"Hubris begets Humility , after much pain." old jim

 

[Averagesupernova]

Asn, you're very close to right about that. However, if you've ever dealt with a cabinet heater thermostat, a 120v motor starter coil in which the overload relay is typically located after the coil (the neutral side), or a four pole break transfer switch, you will find that on some occasions the (utility or system) neutral does in fact get broken. It's not common practice, and as I said as a general rule you're right, but you need to be careful if you're going to invoke NEVER, EVER in the electrical industry.

I have seen cases where the neutral is switched but have chalked it up to poor design practice. However, these cases have been in appliances that plug into a receptacle. It is not necessarily poor practice since sometimes the plug can get reversed. Whenever the neutral is switched in such an appliance steps should be taken so that the hot is also switched. Utility disconnects that I have seen that are a double throw for auxiliary power do not break the neutral.

 

[sophiecentaur]

This Neutral thing is very different on both sides of the Atlantic. They should be given different names, in fact.
Disconnecting a Neutral when not all phases feeding a load have been disconnected is a possible hazard. In UK systems, you have either a three phase supply (big loads) or a single phase supply. Different single phase circuits will fan out from a three phase board and, to isolate one circuit, the L and N are both disconnected, ideally. 'Shared Neutral' is avoided, even within a single household, where only one phase is present so circuits can be truly isolated.
The US domestic system is more complicated and, IMO, more risky as a result. Dual voltages in one hhousehold seems a bad idea, to me.

 

[sophiecentaur]

Presumably the breaker on the incoming supply to a US home is just Double Pole? What happens with fusing ?

 

[jim hardy]

Presumably the breaker on the incoming supply to a US home is just Double Pole? What happens with fusing ?

Yes, 2 pole.Around these parts where there are still a lot of older homes with fuse panels,

the main is a big two position fuse block with a single handle.

Installing it upside-down leaves it disconnected but you don't have to remember where you set it down.

It is not uncommon for one of the two fuses to open circuit from just old age which of course kills the 220 volt appliances and half the lights. Symptoms are confusing because backfeed through a 220 volt appliance can provide limited current to stuff on the "dead" side.

Individual circuits are protected by the little round screw-in fuses. A 220 volt branch would use two round ones or in better panels it'll have another big pull-out fuseblock. In the picture above, one fuseblock is main the other probably for electric range.

Thank goodness those are disappearing as old houses are remodeled or bulldozed. Temptation for the "penny behind the fuse" has burnt down a lot of properties. (Some for the insurance...)

old jim

 

[Averagesupernova]

'Shared Neutral' is avoided, even within a single household, where only one phase is present so circuits can be truly isolated.

I can't see why a neutral would EVER be shared in a residence in the UK since doing so will guarantee the sum of the currents of the 2 hots that are associated with said neutral. The only place sharing a neutral works is multi-phase or split-phase and then has plenty of potential to be done wrong.

The US domestic system is more complicated and, IMO, more risky as a result. Dual voltages in one hhousehold seems a bad idea, to me.

I can't agree. :)

 

[krater]

Utility disconnects that I have seen that are a double throw for auxiliary power do not break the neutral.

You can have a generator that is grounded as a separately derived system, in such case the neutral is connected directly to a grounding electrode system and it is permissable for a transfer from utility to break the neutral, isolating the premesis wiring and running the genset as prime power. I think it's frequently a preference of design and utility standards whether this is acceptable in a given locality. Unless it is desirable to be totally isolated from the utility grid there is no good reason to break the incoming neutral, and as such most transfer switches only break the ungrounded conductors.

 

[sophiecentaur]

I can't see why a neutral would EVER be shared in a residence in the UK since doing so will guarantee the sum of the currents of the 2 hots that are associated with said neutral. The only place sharing a neutral works is multi-phase or split-phase and then has plenty of potential to be done wrong.


I can't agree. :)

It had been done in my home. I'm not saying it should have been and it has now been rectified. There are occasions where the temptation is to use the neutral of a Power circuit as the return for a single switched light. The current imbalance situation would be negligible.

If two phases are used in one premises (180° or 120°) then there is greater probability of the 'wrong' voltage being applied to a unit. There may be good historical reasons for the US system of a split phase arrangement but I have yet to be given a good reason for everything not being fed with 230V. (Actually, there is one reason and that is the unconvincing design of many of the LV mains connectors I have seen in the States - but that's a chicken and egg thing, I think).

 

[Averagesupernova]

Often referred to as 'stealing' a neutral can also result in unnecessary EMFs. The code wants the hot as well as the neutral conductors of a circuit to run in the same cable or conduit. They want them side by side to act as a transmission line. What can often happen is a neutral is lost somewhere on a circuit. When the electrician is called to troubleshoot and repair this situation they may take a shortcut and tie in with a neutral on a different circuit. This can result in a large loop. Think single turn coil. Not only is this frowned upon, it is prohibited due to the EMF thing.
-
Now if I am correct, in the UK don't they run the wire out of the service panel out to each outlet (so-called in the USA even if it is a ceiling light) and then back to the service panel to form what they call a ring? So a load on a wall receptacle will have 2 redundant current paths? Sounds good, but when the one of the conductors becomes open for some reason now there is an unbalance of current in parallel conductors resulting in the EMF that I referred to.

 

[sophiecentaur]

A borrowed or stolen neutral will only happen when someone wants a quick fix in a non-standard situation. I can't see it happening in a high power situation because high power networks have much simpler layouts than lighting circuits. The practice is a bad one - but not for the reasons above, so much as for safety reasons. A borrowed neutral is a potential path between a live circuit and one that has been isolated (in principle) by a double pole breaker. (You will say that the neutral, even of an isolated circuit, should not be broken but what happens with a length of cable that has been left behind because it's difficult to remove during an deinstall? Despite your 'rule' it would still be possible to put your hand on a Neutral coloured wire and find it live.
The issue of dual voltages in complicated (and undocumented) domestic installations is still very relevant, imo. Where you can't be sure of the standard of installation m(historically), it's best to make it as idiot-proof as possible. Single phase within one household is one way towards ensuring that. What is the advantage of the 120/240 system?

[Edit:]The Ring Main system is a pretty good one and it means that many outlets can be hung onto a single 'ring' of twin-and-earth. It uses a 32A fuse and 2.5mm wire, which is enough for heating, electronic equipment etc. etc. on one floor of your average home. It has several advantages - two thin wires are easier to deal with than one thick. A 'star' system requires more total length of cabling and the ring only needs a return length of cable from the last outlet on a 'daisy chain'. It does rely on all plugs having their (device specific) individual internal fuse ( but, if the fusing is done right) the main fuse is rarely taken out due to a device fault. Permanent connections have their own fuses so that there is no load hung directly on the 32A mains fuse. If there is a break in either conductor in the ring, then it may not be detected (about the only disadvantage I can think of). Wiring regs in the UK are very tight and the quality and standard of power distribution in the home is superior to anywhere else, imo. (Not cheap though).

 

[krater]

Sophie, I find it an interesting juxtaposition that you tout the "idiot proof" nature of a single voltage system in a residential setting only to follow up with a paragraph extolling the virtues of a system where each individual device requires correctly sized local fusing instead of simply sizing the entirety of a branch circuit to handle the rating of the source overcurrent device. Not to mention, this arrangement of "ring mains" leaves me questioning its safety with respect to short circuit/ground fault protection. What is the gauge/size of the "spur" wires that lead to the outlets? And is equipment grounding sizing at least continuous with respect to the 32A supply main?

A 120v standard for residential use may in fact trace its origins to a historical design for the first incandescent lamps, however I think it's quite justified considering the many millions of man hours and pieces of equipment it would take to retrofit the entire North American grid with a 230v to ground two-wire service. I think any negligible gain in efficiency would be far offset with this endeavor. The task would have been further complicated in years past when the grounded or "neutral" conductor was also effectively the short-circuit/ground fault protection for the circuit as well as the equipment ground in some cases. In fact a somehwat disturbing number of these installations are still in use today. Also a limit of 120v is currently imposed by code, which does not allow luminaires (light fixtures) or loads under 1440kW (1/4HP) to be connected to a branch circuit over 120v line to line in a space defined as a dwelling unit or similar occupancy.

There is a very valid argument that twice the system voltage simply means double the potential current passing through a human body should one inadvertently find oneself part of an electrical circuit. You can idiot proof all you want, but even in an age where GFI protection for personnel is widely available and affordable people are still killed in the home by electricity. And in such situations where a grounded conductor is connected in place of an equipment ground it's twice the current that is going to try to flow through you instead of the system grounding point should you give it a favorable path. Higher voltages also equal greater arcing potential, higher fault current potentials, and overall greater associated risk.

Higher system voltages are commonplace when you start talking about buildings with hundreds of kW of lighting and several tens of thousands of kW motor loads. In these cases the (many times) cost of installing 480Y/277 or even higher voltage systems is eventually offset by the gains in efficiency; heavy industry often uses 4160v for internal distribution and to feed 100s+ HP motors. And with the growing use of extremely low power equipment such as solid-state and LEDs it is becoming more common to find lighting tracks, emergency lighting, and other hard-wired power and control systems operating at 24v, 12v or even lower, carrying only a handful of amps for tens of feet with building wire sizes that are already common. Greater voltage does not neccessarily equal greater benefit these days even taken from a standpoint of pure efficiency.

While the line of conversation has divereged somewhat it can be brought back to the main point of the OP that in his situation the concern over too high a voltage can probably be considered splitting hairs, just as Sophie, myself, and others could split hairs all day on the advantages of midpoint grounded (note: we still call it single phase over here) versus single voltage systems in residential settings. No matter how great the quality of grid power there is always the chance of transient surges, acts of nature ect, which is why electrical equipment is usually tested and certified some degree beyond what it should ever experience even in likely scenarios of improper use. From a safety standpoint there probably isn't much that has a margin of only a hundred or two volts between normal range and significantly unsafe operation. Much more prevalent are the dangers of currents in excess of what is safe or tolerable; excess volts usually lead to an immediate pop or bang while excess currents fester and lay in wait, until just the right circumstances can cause their revalation in far more destructive ways.

Observe that National Electrical Code is published not by the academic IEEE, but by the National Fire Prevention Association.

Yes. And I'm comfortable standing on this. We don't have 230v plugins cause my code says you can't, Sophie!

 

[sophiecentaur]

That's a nice long list of points. This is not a win - lose argument because there are two successful and established systems that we're discussing.
You are right to point out the risks with the UK plug fuse situation and, for some loony reason, new plugs bought from a shop all seem to be fitted with 13A fuses. That is a serious hazard, in some cases. However, all modern appliances are sold with a moulded plug - and the correct fuse - and the vast majority of them end their lives with the same plug and fuse. This means that the fire risk is small and decreasing. If individual appliances are not protected with their own plug fuse, how, in the US, do you ensure that they are supplied from a current limited socket? The fuse is there to prevent a fire in the flex or the appliance due to over current. If the socket will give them 30A then what happens when a lightweight 5A flex is feeding a 10A 'short' inside the appliance? The ring main system goes a long way to prevent that sort of situation. A ring main supply to a typical living room will have three or four double outlets, or more. Is it the practice to have that number of outlets, separately fed from a central consumer unit? How else can you get proper fuse protection? Sounds like an expensive installation.

Earth leakage protection (residual current) works just fine and is standard for all new installations. The gauge of Earth conductor is set by the Earth wire in the 'twin and earth' cable that is set by the regs. Cookers and water heaters have separate heavy current cables and would not be part of a ring. A shower would have additional RCD protection. Spur wires are not standard in a ring system. The cable loops through each socket to the next and back to the fuse box. Spurs are usually fused and should only power a double outlet (iirc).

Of course, you have to stick with the current system in any country and I can see how they do not permit 230V plugins in the US because that would involve a complete re-design (have you studied the high quality of home produced UK 13A plugs? They are very good compared with most others I have seen - except in Switzerland). The change happened in the UK in the 50's, iirc, when the round pin 15A and 5A sockets were fed in a star system. But people had very few appliances so a couple of outlets in a room were adequate. In those days, people used to have nasty bayonet adaptors to allow small appliances (radios and even a toaster, in my home) to be fed from the hanging lamp in the centre of the room ! (OMG). At one time, to encourage the use of Electric power, there were two meters - one for lighting and one (cheap rate) for power sockets. The ring main system came in with the post war housing development (that's another story). I think the transition was made at a very opportune time when the Government had the strength of influence to make it happen. It would have been much harder for a vast place like the US to have made a similar changeover and it's pretty well impossible now - even if you wanted it.

I always find the discussions about US UK supply differences interesting. It took me a long time to realize that people saying the opposite can often both be correct. lol

 

[MrSparkle]

eh, I feel that most people who have been accidentally zapped by 230V would agree that 120V is the superior system!

 

[sophiecentaur]

I'm not going to get involved in a 'mine's better than yours' argument because both systems have advantages and disadvantages. It is not impossible to get 'zapped' by 230V in the US. The OP seems to suggest that you can expect a poor quality of installation everywhere in the world.

I would like to have an answer to my questions about fusing in the US. The UK ring system works very well and I can't see how you can fuse things safely with a star system, without appliance fusing.

 

[brenfox]

Krater, you have lots of interesting and valid points. If you experienced boots on the ground electrical installation in the UK you will find the emphasis is on good workmanship rather than technical knowledge. This is a pre-requisite of the engineers not the installation electricians. I agree that an average sparky should be able to work out cable calculations for a particular design load but unfortunately this is not the case.

 

[brenfox]

As an after thought (this is where i will get my technical *** whipped!). According to Kirchoff, current entering a node will leave a node in a closed circuit. So whatever current enters a load will leave this load via the neutral in a line to neutral system. The potential voltage will be dropped across the load, so in theory the current will run along the neutral with no volts pushing it?? How does this work?

 

[psparky]

Just a few random thoughts from yours truly in USA setting...240/120 volt single phase panel.

Running two separate lines (2 circuite breakers from phase A for example) from the same bus in the breaker panel would work fine if you run two separate neutrals. Great. If you ran just one neutral, you would overload the neutral and either burn the line or start the house on fire. Not as great.

If you run two different phases and share a nuetral, technically this works and the nuetral never gets overloaded because the phases are out of phase by 180 degrees. However, I believe this shared neutral circuit is now "illegal" in USA...still legal in some other countries sounds like. And yes, if you have 2 phases, you then have the full 240 volts from the panel available in your box if you run line to line.

Yes, UK uses three phase panels in there house. This is great. Yes, USA runs single phase 240/120 system in houses. (pulled from three phase at some point) This is also great. And yes, you can get zapped by 240 volts in your house simply by touching line to line...dont do it, it going to hurt and may even kill ya. I actually wish USA had three phase in their homes, but they don't and amazingly all my devices work just fine on single phase even my 240 volt, 4.8 KVA air conditioning compressor.

Thankfully, all main power distribution in USA is three phase, all commercial and industrial buildings are three phase.

 

[MrSparkle]

As an after thought (this is where i will get my technical *** whipped!). According to Kirchoff, current entering a node will leave a node in a closed circuit. So whatever current enters a load will leave this load via the neutral in a line to neutral system. The potential voltage will be dropped across the load, so in theory the current will run along the neutral with no volts pushing it?? How does this work?

Charges carry energy, they are not the same thing as energy. 1 Volt can be expressed as 1 Joule/Coulomb, meaning that voltage is energy per charge. The current running back to the source on the neutral line has nearly 0 voltage, so it is carrying nearly 0 energy.

 

[brenfox]

psparky, UK households universally use single phase. 3 phase is very rarely used in domestic situations unless the loadings on a big property require 3 phase. (extremely rare)..i think...

 

[krater]

I'll try to briefly address two posts, apologies if I'm dragging conversation further off topic.

If you run two different phases and share a neutral [sic], ...However, I believe this shared neutral circuit is now "illegal" in USA...

It is not illegal, however code now requires that all ungrounded conductors of a multiwire (shared neutral) branch circuit must be disconnected simultaneously by the branch circuit overcurrent device. So where you could at one time have circuits 1 and 22 (L1 and L2 respectively) share the same neutral in single phase, or 1, 3, and 42 (phase A, B, C) in three phase, this is no longer allowable because the new code forces such shared neutrals to be fed from consecutave breaker spaces ie 1, 3, 5...22, 24...ect so that the handles can be mechanically tied together. The device for disconnecting is not required to be a common trip device however it is permissible. Short answer is you need handle ties or multipole breakers in order to share neutrals legally with the latest NEC. This is really great for reducing the risks of getting into a live neutral on a two or three circuit multiwire branch in an installation which complies with the new code. Relating back to OP I would say this, if the engineer wants two circuits to feed the same switching device I would hope that a means would be provided to disconnect both at the same time for servicing.

Sophie, as to you latest on fusing you've more or less got it with appliance fusing. In essence, so long as there is a means within a piece of equipment such as a fuse or self-restoring breaker, or if it contains a motor or transformer source which is inherently protected or power limited, then overload should not create an issue where the conductors in the supply cord pose a fire hazard. As for short circuit and ground fault protection, one must remember that just as an electrical system can experience voltage transients, similarly just because you're connected to a branch circuit protected at 20 or 32 or whatever amps doesn't mean that is all that can get through. Ultimately you're connected to the so-called "infinite bus" and even at point of use on a small residential system a bolted or arcing fault can draw hundreds, even thousands of amps for a few cycles. The thermal-magnetic inverse time circuit breakers you will find in a modern breaker panel have a component (basically an electromagnet) that will trip instantly in this situation regardless of its rating as 15A, 20A, 30A, or otherwise. The relatively rare cicrumstance where an arcing fault creates a conductor overload relatively close to the rating of a circuit breaker is beginning to be addressed with the introduction of AFCI, or arc fault circuit interruptors which analyze the waveform for signatures of an arcing condition and trip the circuit similar to a GFCI.

The point is, over here it's left to the utilization equipment designer to either size their supply cord consistent with the plug configuration of the receptacle it is intended for use with, or provide some internal means to limit the current imposed on the cord during normal and some abnormal circumstances. It sounds to me as if in UK the means is essentially the same; however I see a greater potential for tampering with a fuse contained in a receptacle or cord cap versus one buried on a control board somewhere inside an appliance.

Sorry again if I'm not making enough effort to stay on topic as I'm finding the conversation here quite interesting and informative, I believe we've covered the original concerns of OP but in the process spawned another useful discussion of more generalized subjects somewhat related to the initial question.

 

[Averagesupernova]

If you run two different phases and share a nuetral, technically this works and the nuetral never gets overloaded because the phases are out of phase by 180 degrees.

One thing to note here is this: A purely inductive load that draws 15 amps on one side and a purely capacitive load that draws 15 amps on the other side (can't imagine this in the real world) will put 30 amps on the neutral. This was discussed a while back here on PF. Can't recall what thread. A search will show it up I am sure. Jim Hardy was involved in the discussion as well as myself. It had never occurred to me up to that point that is was indeed possible to overload a neutral. But, it most certainly is.
-
Edit:
Did the search myself, here it is: https://www.physicsforums.com/showthread.php?t=705961

 

[sophiecentaur]

@krater
You mean an internal fuse is supposed to protect the device? That's ok if the flex has capacity in hand. What happens with all those small devices like table lamps and hairdryers; do they all have an internal fuse?
Yes - this is interesting because we are both choosing to find different things important about an electrical supply strategy. I guess the bottom line is that 'not many' people get killed in US or UK so both systems are basically OK.

 

[the_emi_guy]

@sophie & krater

Is it safe to say that the shared circuit breakers within a breaker panel are there to prevent electrical wiring behind walls from overheating and starting fires, but not to prevent individual appliances from burning up due to faults internal to those appliances. The latter would be covered product safety standards which impose their own fusing requirements.