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How avoid overvoltages when there is loss of neutral in electricity supply system?

Hi,
As you know, all electricity from your national grid is ultimately three phase in origin. If the neutral gets disconnected somewhere in the supply, then the Line-Neutral voltage becomes the Line-Line voltage, and any connected devices may well get overvoltaged and blow up.

For a 10kW installation, supplied with three phase, how can we ensure that the Line-Neutral voltage does not rise up to the line-line voltage if there is a loss of neutral connection?
 

MaxHeadRoom78

Well-Known Member
Hi,
As you know, all electricity from your national grid is ultimately three phase in origin.
Except residential supply which 'ultimately' ends up as 1ph centre tapped transformer.
In this case, if all 1ph 120v circuits have their independent neutral (i.e. not shared) then L1 & L2 with N loads should not end up with full 240v or whatever the
unbalance result is.
Or are you refering to an industrial 3 phase supply?
Max.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
You follow the proper wiring and installation guidelines!

And three options; worst to best:
You include in the warranty documentation that faults in the supply to the equipment are not covered, and charge for repairs.

You add a monitoring circuit that only switches power on to the rest of the system if the phase-phase and phase-neutral voltages are all correct.

You eliminate the neutral and make all loads balanced phase-phase, optionally with phase monitoring and fault indication.
There is no good reason to have a neutral in properly designed three phase gear.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Except residential supply which 'ultimately' ends up as 1ph centre tapped transformer.
In this case, if all 1ph 120v circuits have their independent neutral (i.e. not shared) then L1 & L2 with N loads should not end up with full 240v or whatever the
unbalance result is.
Or are you refering to an industrial 3 phase supply?
Max.
You're referring to the wrong country, he's in the UK
 

gophert

Well-Known Member
Most Helpful Member
Hi,
As you know, all electricity from your national grid is ultimately three phase in origin. If the neutral gets disconnected somewhere in the supply, then the Line-Neutral voltage becomes the Line-Line voltage, and any connected devices may well get overvoltaged and blow up.

For a 10kW installation, supplied with three phase, how can we ensure that the Line-Neutral voltage does not rise up to the line-line voltage if there is a loss of neutral connection?
Is this a common problem in the UK? If so, I'm surprised nobody else has solved the problem (or even asked about the problem). If is not a common problem, is it at least a known problem? What advise does the grid operator have? What liability do they have? Do certification companies (UL, CE, TUV, ...) have a test standard for dealing with such cases? What is the pass fail criteria for their tests? Must the device simply avoid catching fire and burning down buildings? Must the device survive unharmed? Something in between (replace a fuse)? How do other top-tier UK electric/electronics companies handle this (oh, wait, I don't think any top-tier UK electric/electronic companies exist - ignore last comment). I assume there are all kinds of electrical problems 8n the UK since the power plugs must be 5x bigger than any other European country uses.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
Is this a common problem in the UK?
Only possibly with industrial gear that has a flying lead and plug.

A lot of places do not take good care of flexible cables, especially power extensions, and it's not unusual to see them pulled out of the cable clamps and the individual core insulation showing. That of course means all the tension is on the individual wires and broken connections are possible.

I've seen quite a few faults and industrial machines messed up or damaged due to that type of thing.


Re. your usual daft/insult comments - have you ever actually seen a UK standard power plug? See the photo below: US, European and UK grounded plugs side by side... Not a lot of difference.

Plugs_800.jpg
 

unclejed613

Well-Known Member
Most Helpful Member
We certainly don't get the crazy cascading power cuts the USA does.
somewhere a few years back i found a real time map of phase lead and lag across the whole US grid system... apparently most of it runs synchronously, and any severe lead or lag can cause problems on large areas of the network. so i looked up how they maintain the synchronization, and as it turns out the primary method is simply Back EMF... there are also phase balancers etc... in the system. i think one of the reasons there are so many more failures in the US system is the huge land mass and long distance lines here, plus there are a LOT of power statins feeding the national grid.... more moving parts and more to go wrong.... plus the occasional terrorist squirrel crawling around or climbing where it shouldn't. https://cybersquirrel1.com/ select "squirrel" on the search function on the map and look at the density of squirrel induced failures in the USA compared to other parts of the world... of course this map includes all infrastructure damage... not just power systems...
 
Last edited:
Or are you refering to an industrial 3 phase supply?
I dont really know...just a "three phase supply" where each phase is 240Vrms.

I believe most of UK is ultimately supplied like this.

I am amazed i cant find examples of damage to household electrical goods due to "lost neutral"...because most goods, even industrial ones, are not designed to be able to handle the ~587V peak they will get if the "lost neutral" problem happens.
 

unclejed613

Well-Known Member
Most Helpful Member
there have been some examples in split phase (single phase with the two hots 180deg out of phase) where an open neutral or a neutral with a high resistance connection to the entry panel can cause serious problems such as overvoltage on one half when a heavy load (like a furnace motor or clothes drier motor) is placed on the other half.... if it's a high resistance connection to the neutral, it can also be a fire hazard.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
I am amazed i cant find examples of damage to household electrical goods due to "lost neutral"
Remember the Uk uses the "Protective Multiple Earth" wiring scheme.

Neutral and earth are directly bonded together at the incoming connection, and at each substation. Losing the neutral is no different to losing a live with that setup, continuity is lost rather than there being an overvoltage & anything with an earth leakage trip should disconnect.

The substation transformer secondary star point would have to become totally disconnected for there to be a voltage difference between live phases and the external neutral/earth.

That's a fundamental part of the fixed equipment so extremely unlikely to occur.

The risk is with plug-in three phase systems and flexible cables, as I said to start with - and on those, there is no really good reason to run things live-neutral rather than live-live.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I dont really know...just a "three phase supply" where each phase is 240Vrms.

I believe most of UK is ultimately supplied like this.

I am amazed i cant find examples of damage to household electrical goods due to "lost neutral"...because most goods, even industrial ones, are not designed to be able to handle the ~587V peak they will get if the "lost neutral" problem happens.
Why would you imagine any such thing? - why would you think losing the neutral would 'magically' somehow produce 587V on the single wire remaining?
 

Ramussons

Active Member
3 Phase Residential supply is very common in my country. And I have come across quite a few cases where the Neutral Link in a 4 pole isolator has failed leading to a situation the TS refers to.
I think the only solution is to monitor each Phase -Neutral for Over voltage and shut off the faulty Phase(s).
 

Ian Rogers

User Extraordinaire
Forum Supporter
Most Helpful Member
You should all know by now, As the OP hasn't grasped the basic concepts, weird questions are asked... Oh! My three phase equipment DONT EVEN USE NEUTRAL!!!

4 wires Red, Yellow, Blue ( 3 phase) and earth... See if you can get your head around that..

I'm more concerned about loosing a phase than earth or neutral....
 

Nigel Goodwin

Super Moderator
Most Helpful Member
3 Phase Residential supply is very common in my country. And I have come across quite a few cases where the Neutral Link in a 4 pole isolator has failed leading to a situation the TS refers to.
I think the only solution is to monitor each Phase -Neutral for Over voltage and shut off the faulty Phase(s).
Except he's asking about UK single phase supplies to domestic premises - so only two incoming wires, neutral and live - if either is disconnected you get no power at all.

Industrial three phase is somewhat different, and can be connected as either star or delta - where delta has no neutral at all. But any three phase system should have little current through the neutral anyway, as it's supposed to balanced across all three phases as much as possible, hence thick live wires and only a thin neutral.
 

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