Losing neutral in the utility system

[sophiecentaur]

. . . . and I am right that at least someone should have spotted the dimming lights or groaning motors - which must have been powered, to account for the vast voltage drop.
Someone should have had a real roasting about that.

 

[jim hardy]

BTW for what little this is worth
Going back to the events of post #19, incoming neutral and hot wires swapped at panel, placing 240 volts across half the circuits in my neighbor Harry's house...
Yesterday i replaced his refrigerator's freezer fan . It had burnt up in the minute or two we had 240 on it. Surprisingly it still made ice cubes but the lower compartment wasn't getting cold enough.
There was a mild buildup of ice on the evaporator so I suspect we'll have to replace the defrost timer too ... Compressor and evaporator fan are thermally protected thank goodness.

 

[jim hardy]

. . . . and I am right that at least someone should have spotted the dimming lights or groaning motors - which must have been powered, to account for the vast voltage drop.

You'd sure think so.
Universal motors with brushes are more tolerant of out-of-spec voltage than induction motors.
My retired carpenter friends reminisce about running their 110 volt worm drive Skilsaws on 220 . "They really go!" .
Ahh youth.

Induction motors will burn up from either over or under voltage.

 

[krater]

Diagram of this not exactly open neutral situation

The red lines are circuit conductors L, N... the very faint green line after the service point is the equipment grounding conductor or protective earth. It existed everywhere along that path, I don't believe they were ever connected together again anywhere down the line, yes it was some time ago but that hasn't been permissible for some time either.

As for noticing any other effects of the situation, well, apart from flickering fluorescents at the main trailer there simply weren't many lights connected nor were there that many non-intermittent motors as this was a temporary construction service for a 98% concrete structure.

 

[sophiecentaur]

A nightmare!

 

[krater]

From the fact that you seem to regard the Earth conductor as a possible backup for the neutral circuit

You misunderstand. The closest thing I've said to that is probably the fact that the neutral could concievably be a backup (or replacement) to ground. That was once standard (cost related) and still exists regularly in dwellings. Scary. Here's the thing, and yes it's been alluded to already, so long as circumstances, for example wood construction and dryness, ensure that one remains isolated the system works just fine to prevent ground faults from starting a fire with little or no potential for shock.

I can see by your many many citings of RCDs that you are very comfortable in total reliance on them. Unless they are priced at hundreds of dollars apiece I judge your trust as poorly placed. Once again, do you TEST your RCD? Ever? How old are they? Do you have any idea how much GFCIs and the standards behind them have changed in NA just in the last decade? How do you know it will detect any leak in the neutral, and trip in such a condition, instead of happily frying you if you somehow end up solidly connected between neutral and local ground potential?

Consider a situation where your utility loses your PE connection in the circuit that serves your electrical works*. Maybe with a mostly underground system that's difficult to imagine but we will consider it since a great many power lines still exist on poles. You don't own or control the PE connection so as far as maintaining it you are at the mercy of the utility. Even if you were in an underground situation in NA this is not a situation you ever have to worry about. Why is that? Because at this point of the system the "PE" is the NEUTRAL conductor! Yes, it's the zero potential wire but since it's zero it is used as a fault current path for non-circuit metal parts as well. So when the neutral connection is lost on a UK system, the lights go out. When the neutral connection is opened on a NA system, well, something is likely to go, somewhere. A loss of PE before the service is tantamount to a loss of neutral in the NA system. But when the utility side PE goes out in a UK system, nothing happens!

This can have serious ramifications aside from the fact that it occurs without indication. Since on the system a circuit conductor exists which is zero somewhere is in close proximity to a conductor which is zero locally there is the chance of a fault developing between them. The fault will likely happen at a low potential since zeros might be relatively close. But low potential might also create the balance of a current sufficient to start a fire but too low to clear a main. The RCD is intended to prevent this but unless it is self-diagnosing with a fail-safe design it is a single point failure in the protection chain under these circumstances.

I will state this again, if you can't guarantee that a zero will not become more than zero the safest way to handle it is to connect it to as local a zero as possible. Your system gives up on that a step before ours. Probably on cost with no real consideration beyond that. Maybe it's not a big deal there; the world is a bit smaller. It can be a big deal here. As far as knowing where the standards come from, well it's part of continued training that I attend for example informationals put on by the people who are, yes, developing and writing the standards, they are not some inaccessable oracle in a far off high-tower but are in fact regular people like you and me who don't hide their reasoning behind some smokescreen.

The statement that with no faults there will be no trips is idealized, yes. I'm familiar with the devices and their uses, yes they do nuisance trip in all kinds of situations. But you don't appreciate that 5mA point of use devices may be much more abundand than probably even RCDs, and nobody deems them useless due to spurious trips. 30mA range devices are used here, too. They are only permitted to be applied in situations where human safety is not the reason for the ground fault detection, it is an equipment protection standard to keep monster sparks from shooting out of things. Equipment protection is common in another range, the 1000A+ devices that need big overgrown RCD/GFCI circuits to keep a high impedance fault from burning down the gear without ever drawing enough amps to trip a thousands of amp device set to accommodate a large building's inrush.

And I don't consider it to be bringing in extra issues when I'm illustrating my side of an argument with the full implementation of the system I'm favoring. Just as RCDs are integral to the safe function of your side, GFCIs are necessary and required at times on my side. And you still seem to be ignoring my presentation of another reason why the more local the grounding, the better: fault/surge energy dissipation, in other words, abnormal circuit conditions that you seem intent to lump in with Earth return circuiting.

*"Service" or "Service point" = The spot where your main OCP is in UK, the spot where your main OCP and last neutral earthing is in NA. Usually at this point "you own it"

edit: zero will NOT become more than zero

 

[jim hardy]

Diagram of this not exactly open neutral situation

The red lines are circuit conductors L, N... the very faint green line after the service point is the equipment grounding conductor or protective earth. It existed everywhere along that path, I don't believe they were ever connected together again anywhere down the line, yes it was some time ago but that hasn't been permissible for some time either.

Thanks !

Good one . Ground troubles give really confusing symptoms.

An open near originating end in that thousand foot grounding conductor would let it float to somewhere between L and N by leakage, as you suggested.

I've been known to test grounding conductor's ability to clear a fault by applying one, but only downstream of a 15 or 20 amp breaker. Wouldn't do that on this one though for fear of shocking somebody back in that trailer. I will though check for voltage between L and N unloaded, then risk sacrificing a cheap analog multimeter by reading resistance L to N. That gives me confidence the green conductor is continuous. Where they use conduit for grounding conductor , loose fittings can bite you.

Thanks for sharing your mystery Krater. I love anecdotes about troubleshooting .

old jim

 

[krater]

The equipment ground was isolated at the metal-encased hydraulic stressing machine, leaving the metal case and any metal connected to "float". I checked for N-G short in the faulty machine, and found nothing (using a DVOM anyway...yes i know). The serving circuit seemed to be working properly otherwise, although for additional safety reasons a majority of construction equipment is now put together in a double-insulated package to help further protect against just these sort of conditions.

By the time I got up there to check the problem the crew had gotten another identical stressing machine that was reportedly working fine, upon examination I found that the new machine had already gotten its equipment grounding connection removed from the metal case. Funny coincidence, or maybe not so coincidental at all considering the usual environment.

This wasn't the only exceptional incident I observed at this particular site. One humid day in early summer I witnessed anomalous behavior of small bits of 18ga steel wire, a few inches long, which spontaneously exhibited magnetic behavior in contact with some of the installed structural rebar; it would become attracted to the tip of a non-magnetized metal tool and could be led around like a dog on a leash. Nobody had a good explanation for that one and I've never seen the phenomenon repeated.

 

[sophiecentaur]

Just as RCDs are integral to the safe function of your side

No they're not the only safety measure; they are only an extra backup against some faults and will also draw attention to them. If the whole of the exposed metal structure of the house / site are at the same potential then any accidental path from Live to Earth (not even enough to blow a fuse) the potential on any exposed metal will always be the same as you when you touch it. Whether the neutral and Earth are connected, this will be true. RCDs are relatively new devices and hordes of people didn't die previously when their electric kettle element corroded through from L to the Case.
I think you are basing many of your ideas on non domestic situations. What goes on 'on site' can be far worse because people take short cuts (illegally) and bend the rules but I am surprised at the number of serious problems that have been quoted due to what appears to have been plain bad practice.
I take your point about the Neutral being intended as a 'fall back' for the Earth but the risk of shock is pretty low if there is good quality connection of all exposed metal to the local 'Earth'. network What happens down the road would not be relevant except for supply maintenance staff who would presumably follow proper safety procedures when arriving at a site.

 

[darth boozer]

Just for info, in the early 1980s I worked in an RF screened room with hefty filtering on the 240V supply. While trying to trace the source of some RF interference, I tested the mains supply in the room and was able to draw about 7 amps at 14 volts between neutral and the earthed walls of the room.

 

[sophiecentaur]

Just for info, in the early 1980s I worked in an RF screened room with hefty filtering on the 240V supply. While trying to trace the source of some RF interference, I tested the mains supply in the room and was able to draw about 7 amps at 14 volts between neutral and the earthed walls of the room.

I wonder who paid for the energy.

 

[sophiecentaur]

Just for info, in the early 1980s I worked in an RF screened room with hefty filtering on the 240V supply. While trying to trace the source of some RF interference, I tested the mains supply in the room and was able to draw about 7 amps at 14 volts between neutral and the earthed walls of the room.

I imagine that the 14V Neutral voltage was due to the internal resistance of the 'hefty filtering' - i.e. the current supplied to the equipment inside the room was enough to cause a 14V drop through the N leg of the filter. Was there 14V on the N wire, upstream of the Room?

 

[Guineafowl]

I realize this argument is months old now, but I must point out a few (hopefully interesting) things...

5mA is a personnel protection standard widely recognized in NA and elsewhere, from what I'm seeing your equal is 30mA and we are talking at twice the voltage, meaning it can get through twice the path resistance. Ventricular fibulation occurs between 50-100mA and this is why in NA the 30mA standard is considered for equipment protection only and is insufficient to protect people from electric shock. You call me xenophobic, I call you reckless and where does it get us? But go ahead and argue how a smaller tolerance is somehow less safe.

Now, of course a lower fault current limit is safer. But there’s a reason for the 5/30 difference, and I think I see it...

And secondly, do you not realize that scores and scores of 5mA GFCI devices are in use every day, even on things like vending machines? What, do you think someone has to go around and reset GFCIs every week or two so that they can ensure their sodas are still selling?

I assume this is about a dedicated GFCI for each machine? If so, there’s your difference. UK circuits are usually protected in two groups fed by an RCD each. We’re probably going to move to RCBOs for each circuit one day, but that’s the way it is now. In an ideal world, those RCDs should be 5 mA, but this is reality, and earth leakages, even from new equipment, add up, and at 240V will be twice that seen in the US. Indeed, with a peak voltage of 330V or so, there may be dielectric breakdown twice per cycle that wouldn’t happen at 120V at all. You say 30 mA is a dangerous threshold, and it may be, but the real-world shock current will be the difference between the ‘normal’ leakage and the threshold.

A 5 mA RCD protecting a ring main, an immersion heater, lights, a shower... I bet you any money this will ‘nuisance trip’. This is extremely annoying. And if you squeeze safety standards too hard, what happens? At least one person out of the 70 million here will bypass it.

A final point: We’ve had the dual 30 mA RCD system here for some time now, so we’ve ‘sucking and seeing’ for a while. If a single death occurred despite the RCD working as it should, there’d be an inquest and a call for revised standards. I challenge you to find such an incident, because I can’t. The 30 mA threshold is there not because we think a shock below this is perfectly healthy, but to achieve a balance between safety and usability. As it stands, nuisance tripping is rare, and fatal shocks are rare.

 

[krater]

Now, of course a lower fault current limit is safer. But there’s a reason for the 5/30 difference, and I think I see it...earth leakages, even from new equipment, add up, and at 240V will be twice that seen in the US. Indeed, with a peak voltage of 330V or so, there may be dielectric breakdown twice per cycle that wouldn’t happen at 120V at all...

Yes I suppose this was somewhat addressed in above discussion when I believe I mentioned that 120V to ground is a maximum exposure safety standard for operators per ANSI/OSHA standards for many industries. Many, many control and operation functions that used to be carried out in potentially hazardous environments through 120V circuits are now using 24VDC or less as a control voltage, with a power limited source to further increase safety.

I have seen three or four 120V GFCI devices used for temporary power during building construction, many of which were split initally at what we call a duplex receptacle, many plugins of which had either a long extension cord or a three-way pigtail splitter splitter, or both or even other combinations thereof plugged into them, which ran multiple battery chargers/lights/electric hand tools fed from cumulatively hundreds of feet of cord. And with proper attention paid to the installation and the conditions of the cords or tools connected to them, there are surprisingly few problems with spurious tripping of the protective devices. Nearly every time it happens it's either related to a damaged cord that when removed solves the problem, or some more complicated situation involving something like a large motor that can often be resolved by re-arranging a few things.

 

[Guineafowl]

Yes I suppose this was somewhat addressed in above discussion when I believe I mentioned that 120V to ground is a maximum exposure safety standard for operators per ANSI/OSHA standards for many industries. Many, many control and operation functions that used to be carried out in potentially hazardous environments through 120V circuits are now using 24VDC or less as a control voltage, with a power limited source to further increase safety.

I have seen three or four 120V GFCI devices used for temporary power during building construction, many of which were split initally at what we call a duplex receptacle, many plugins of which had either a long extension cord or a three-way pigtail splitter splitter, or both or even other combinations thereof plugged into them, which ran multiple battery chargers/lights/electric hand tools fed from cumulatively hundreds of feet of cord. And with proper attention paid to the installation and the conditions of the cords or tools connected to them, there are surprisingly few problems with spurious tripping of the protective devices. Nearly every time it happens it's either related to a damaged cord that when removed solves the problem, or some more complicated situation involving something like a large motor that can often be resolved by re-arranging a few things.

Ah. On building sites, a site transformer is often used, stepping the 240 V down to 110V with a centre tapped PEN. This means an operator is only exposed to the potential between one tap and the PEN, i.e. 55V.

My mate’s oven was recently nuisance tripping the RCD. As it cooled, a relay would click and sometimes, not always, the RCD would go. I measured the peak fault current using an adaptor to allow a current clamp around the live and neutral combined. It never went above 30 mA, more like 9 or 10. I guess here, the whole house Earth leakage (an old supply, with one RCD protecting everything - crap) was topping up the oven’s contribution. Either that or the old RCD’s threshold had drifted. Anyway, the oven was brand new so it went back under warranty and there have been no problems since.

If your Earth loop impedance is low enough, you may have some circuits not under RCD protection - this is common for ovens. Otherwise, it’s RCDs everywhere here.

 

[Guineafowl]

proper attention paid to the installation and the conditions of the cords or tools connected to them

Ha ha ha (hollow laugh).

You haven’t seen some of the things I’ve seen, man. You weren’t there, man.

1. Plug fuses bridged with foil, wire, or a segment of six-inch nail. (“It keeps blowing, so I made another one”).
2. Scuffed cables with live exposed. (“Watch that cable - it’s a bit dodgy”).
3. Twisted-together wires, wrapped with tape and lying on a wet lawn.
4. Above half-submerged in a small puddle.
5. Damaged plugs where the back comes away when you try to pull it out, exposing all connections.
6. Metal light fittings that “keep tripping the RCD”, so they remove the Earth wire, and allow the casing to float happliy.
7. A dishwasher I bought from a friend - “turn it off before you open it - the metalwork gives you a tingle” - leaky EMI cap across input. Actually it was blown almost to bits.

Etc.

There’s always a danger that too safe a system results in blunted danger perception.

 

[krater]

Ha ha ha (hollow laugh).

You haven’t seen some of the things I’ve seen, man. You weren’t there, man.

Oh come on now. All these sound pretty tame and don't involve conductors the size of your thumb or noises similar to a freight train happeningn inside equipment. Those of us who are intimate with the animal are by and large very aware of the risk. The remainder of the populace is almost totally oblivious to the hazard. Herein lies the approach of applying safety standards.

 

[Guineafowl]

Oh come on now. All these sound pretty tame and don't involve conductors the size of your thumb or noises similar to a freight train happeningn inside equipment. Those of us who are intimate with the animal are by and large very aware of the risk. The remainder of the populace is almost totally oblivious to the hazard. Herein lies the approach of applying safety standards.

I’d say the vast majority of people, at least in the UK, are indeed oblivious to the ins and outs of electricity. As such, they view it as a kind of dangerous black magic, and won’t dare to fiddle. Hence the many call-outs electricians get to simply reset someone’s MCB.

Farmers.

By and large, the salt of the earth. Problems:

1. They are used to doing things for themselves.
2. They need things done now, so they can get on.
3. They have piles of junk that can be used to cobble together solutions to problems.
4. They are used to working with clapped-out gear.
5. They are practical people with tools and a little knowledge.

You’ll always find the worst, excremental electrical setups on farms. If that’s not a testament to the 30mA threshold, I don’t know what is.