Understanding Neutral Circuits: Loads & Current Flow

[Crazyhorse2882]

In a single circuit I've always understood that a neutral was a return essentially completing the circuit for the current to flow but I know that losing a neutral can cause imbalanced load and burn up equipment, my question is, how is this possible? If I were to open the hot, it would simply open the circuit and stop the current from flowing, so my thinking is that lifting the neutral is the exact same thing, that it would just open the circuit and no current would flow no different from opening a light switch. Another question I have is, many people refer to a load as "drawing the current" where I've always viewed it more as completing the circuit so that current can flow, which is the better way to view it? Thank you

 

[Windadct]

Open Neutril in 3 phase causes problems because the the other phases MAY create a different circuit configuraiton, you would have to sketch this out to see the issues, and it depends on where the neutral break is.

As for single phase, it will also depend on where in the total circuit the break is and what type of break it is, if is not a clean break, or worst case probably and arcing break the voltage to the appliance can be low and low erratic. - As a short summary - it depends on the location and type of open circuit -

personally I hate the "drawing current" this implies some actopn on the load side ( like pulls current)- I prefer language like " allows current" The utility is "pushing" the load is ether off or on, but not pulling. - Perhaps a subtle point.

 

[TurtleMeister]

As Windadct indicated, the effect of an open in the neutral line depends on where the open is located. The illustration below shows 5 locations. 2 in the hot line (A and C) and 3 in the neutral line (B, D, and E).

Only one of them would cause a problem. An open at location B would cause the two loads to be connected in series across the 220 volt line. Since the 50 watt load will have a higher resistance, it will see a higher voltage than the 100 watt load. This is the imbalance that you referred to in your OP. This could cause the 50 watt load to be destroyed, or even worse, cause a fire. This is a particularly dangerous situation because the breakers and ground fault would not protect you.

This actually happened to a coworker of mine. Several devices in his home were destroyed. But I think the power company paid for it because the fault was actually located outside his home, at or near the transformer.

 

[sophiecentaur]

This is a particularly dangerous situation because the breakers and ground fault would not protect you.

This actually happened to a coworker of mine. Several devices in his home were destroyed. But I think the power company paid for it because the fault was actually located outside his home, at or near the transformer.

If the neutral goes open circuit then there would be instant protection from the RCD breakers in the supply - as there would be an enormous difference between phase and neutral currents. Obviously, an old fashioned 'earth leakage trip' wouldn't be aware of anything wrong but I don't think they are used much these days.
In the UK, domestic users are mostly given one phase of the three (i.e 230V and a Neutral). The US split phase system is not used at all and the only time an individual user is supplied with multi-phases, is when it gets all three phases for heavy loads, as in an office or factory.
This, from Crazyhorse

In a single circuit I've always understood that a neutral was a return essentially completing the circuit for the current to flow

needs a comment, I think.
The neutral has precisely the same 'status' as the live conductor and isn't any more of a return path then the live. It's just a circuit. Where two or more phases use the same neutral, Kirchoff tells us that the net current in the neutral can be zero, when the loads are balanced - but that's only at points 'upstream' of where the separate circuits come together.

 

[Crazyhorse2882]

Thanks everybody. Thanks turtlemeister that drawing really helped!

 

[Windadct]

Sophie - what is an RCD Breaker ? I did google, no luck. Residual Current Detector? May be just be a naming difference between the UK/USA - Here in the USA - these are not applied at the residential distribution level, although GFCI ( Ground Fault Circuit Interrupter ) work on this principal and are now required for some branch circuits in the home, but often receptacle based in areas with water present - or the newer AFCI (Arc Fault Circuit Interuptor) is needed in certain new construction - these can be a pain because they detect frequency and some items like treadmills or larger appliances with drives may set them off.

 

[NascentOxygen]

Residual Current Detector?

This trips if it detects a difference* between the current in the neutral and that in the active line.

* typically 20mA or more difference is enough to cause it to trip

In many fault conditions, including many electrocutions, current gets diverted to ground, leaving the neutral to carry less current than the active line and the breaker trips very quickly.

 

[dlgoff]

In the United States and Canada, a residual-current device is most commonly known as a Ground Fault Circuit Interrupter (GFCI), Ground Fault Interrupter (GFI)...

http://en.wikipedia.org/wiki/Residual-current_device

 

[Windadct]

That is what I thought - ( I out - I in > 0 = Residual current ) - do they apply at the local source? (transformer secondary breaker - of course) -- seems like it would cause a lot of nuisance trips.

 

[sophiecentaur]

RCDs are put on the single phase supply - either for the whole 'consumer unit' or for individual circuits (Phase and neutral in - phase and neutral out - with four terminals). They are pretty good because they measure the difference between currents in the phase and the neutral. Unless there is a path to Earth (usual fault condition) or the path to neutral is disconnected (this horrific scenario) the two currents are equal to within a very few mA. The do a good job of detecting damp in cables as well as conduction through people's bodies.

 

[TurtleMeister]

It was the outlet type GFI units that I was referring to in post #3. I've never seen RCD breakers used in an electrical service panel in the US. But I haven't seen any new installations for quite a long time.

 

[Averagesupernova]

As far as I know, arc fault breakers also protect like a GFCI does. I have never tried it, but does a GFCI outlet or GFCI breaker require a neutral in order to work? They were designed to detect imbalance of current downstream from them (through them) which has to occur during 99.9% of electric shocks. Does losing neutral upstream guarantee it will open? I would not bet on it.

 

[sophiecentaur]

As far as I know, arc fault breakers also protect like a GFCI does. I have never tried it, but does a GFCI outlet or GFCI breaker require a neutral in order to work? They were designed to detect imbalance of current downstream from them (through them) which has to occur during 99.9% of electric shocks. Does losing neutral upstream guarantee it will open? I would not bet on it.

Doesn't it go without saying that, if current is not flowing in the neutral, the thing will trip? Any imbalance will do it.

 

[Averagesupernova]

You aren't getting my point sophie. I suspect that a neutral wire feeding the GFCI outlet is required to get the outlet to trip when there is a current imbalance. I could be wrong, but it is worth mentioning. If the neutral is completely gone, then the GFCI outlet isn't 'powered up' so to speak. I can pick up a GFCI outlet off of the shelf, push the button into reset it and there is continuity from the source hot terminal and the narrow slot of the outlet. So if this were the only wire connected to the GFCI, is it able to trip when it normally would when correctly installed? The cause of a GFCI tripping is about always downstream of the GFCI where the safety concern generally is.

 

[sophiecentaur]

I don't think you are understanding how a RCD works in the UK system. It is a double pole isolator with four terminals which opens when there are different currents in live and neutral lines. How could that not work? A simple trip which measures Earth currents directly would obviously give no protection if the current could take a non-earth path. Such devices were used, back in the 60s in the UK - I know, as we had one. But the UK moved on since then; starting in the late 60s' I know because as I bought an RCD back then.
AhA! Of course, I just realized: with a split phase system (US), the neutral current can easily not be the same as the live current. So an RCD would disconnect the supply as soon as there are appliances connected between the two anti phase lines. Which is why the US doesn't use them. This is yet another example of US /European misunderstandings about Mains electricity.
Electrical suppliers in the UK have RCDs "off the shelf" and all new installations use them. If (God forbid) the neutral came disconnected back at the transformer then two adjacent houses would each have RDC trips and one or both of them would instantly disconnect. It is not permitted to connect between phases where single phases are supplied.

 

[Averagesupernova]

I need to research specifically the differences in an RCD in the UK and GFCI outlets and breakers we use in the U.S. I suspect there are not that many differences if any. I am very familiar with what happens here in the U.S. after the transformer. In the UK, not at all. I have done lots and lots of permanent mains wiring. I will post more later.

 

[sophiecentaur]

I need to research specifically the differences in an RCD in the UK and GFCI outlets and breakers we use in the U.S. I suspect there are not that many differences if any. I am very familiar with what happens here in the U.S. after the transformer. In the UK, not at all. I have done lots and lots of permanent mains wiring. I will post more later.

The RCD can only work when you can rely on the same current through both wires. It could work for individual outlets in the US system and anywhere after the neutral is 'split'. It would not be usable when the neutral current could be ' diverted' through the other phase, though.
We do live in an interesting world, though. Things that you take for granted are so different elsewhere. Did you know that, in the US and certain other places, they even drive on the wrong side of the road? And, my dear, the language. Apparently, the word "pavement", in the states, is where the cars go. In the UK, it's the place where the pedestrians walk. I have a bi-lingual (US/UK) grandson and they have great difficulty deciding what words he should use in instances like that. With luck, he will avoid becoming an Electrical Engineer (and earn a lot more money) so phases may never cause him worry.

 

[sophiecentaur]

I just looked at Wiki and they make multi-phase rcds, too. But that wouldn't help the disconnected neutral scenario.

 

[Windadct]

The RCD will not help the case you discussed for multiple phases or split phase with broken neutral, because all of the current returns on the normal conductors ( i.e. not ground). This type of failure is not what this protection is set to do.

 

[Averagesupernova]

Looks to me like the operation of a GFCI and an RCD is identical. However, they can be incorporated into many different devices. The most common type in the U.S. is in a receptacle. One receptacle at the beginning of the circuit can be wired to protect the rest of the circuit. The loss of a neutral between the transformer and the meter will NOT cause a GFCI incorporated into a receptacle to trip due to imbalance of current because there won't be an imbalance. The case of the GFCI built into a circuit breaker works in exactly the same way. Losing the neutral is ALWAYS bad news concerning safety as well as harming equipment. There is a good reason that the U.S. code prohibits interrupting the neutral in ALL cases. The neutral is never, ever, under any circumstance, no matter what, interrupted.