Voltage spike causing strange readings?

[MdubElec]

Hi guys,

Our client called us out to assess a fault in one of their buildings today, it turns out that certain circuits (those with fixed electronic equipment within them) are now acting as if there is no load and the neutral wires back at the board are now showing 230V to earth.

Could a voltage spike be the culprit of absolutely frazzling these pieces of equipment to the point that they could cause very strange behaviour?

If so the client is not going to be happy after tomorrows in depth investigation concludes that most of their equipment needs replacing.

Thanks in advance for any replies.

 

[MDJ]

are these readings on the neutrals on three phase boards.

 

[MdubElec]

are these readings on the neutrals on three phase boards.

Yes they are.

 

[MDJ]

Yes they are.

so you have single phase circuits connected to a three phase distribution board and the neutrals in the board are measuring 230 volts to earth? if so I reckon, and of course could be wrong, you have a hung neutral. Is the neutral to the board through the main switch and switchable or a solid link mate

 

[MdubElec]

so you have single phase circuits connected to a three phase distribution board and the neutrals in the board are measuring 230 volts to earth? if so I reckon, and of course could be wrong, you have a hung neutral. Is the neutral to the board through the main switch and switchable or a solid link mate

I'm not sure it was one of my electricians there today. I assumed it was a lost neutral from what I was hearing at first. What do you mean by "hung" neutral? It's an old style hager board so I think from memory those are screw type terminals for the incoming neutral.

 

[MDJ]

if the neutral is switched in the board or via a contactor for example and it fails you will put 230 volts to earth via the neutral and 400 volts across the 230 volt circuits and will burn out all electronic equipment. this can happen and is why you usually never switch the neutral in TPN systems where single phase circuits are connected, I have seen this several times in my career, it may not be the case, and I am only guessing in this case but get your sparky to check if the neutral is switched somewhere between the MCCB panel board to the distribution board. If on this occasion it is a hung neutral then there could be trouble ahead mate, I hope it is not this

 

[MDJ]

I'm not sure it was one of my electricians there today. I assumed it was a lost neutral from what I was hearing at first. What do you mean by "hung" neutral? It's an old style hager board so I think from memory those are screw type terminals for the incoming neutral.

if the neutral goes straight to the bar and not via a 4 pole switch you are probably okay and it is something else mate.

 

[UNG]

Hi guys,

Our client called us out to assess a fault in one of their buildings today, it turns out that certain circuits (those with fixed electronic equipment within them) are now acting as if there is no load and the neutral wires back at the board are now showing 230V to earth.

Could a voltage spike be the culprit of absolutely frazzling these pieces of equipment to the point that they could cause very strange behaviour?

If so the client is not going to be happy after tomorrows in depth investigation concludes that most of their equipment needs replacing.

Thanks in advance for any replies.

Have you checked for a floating neutral, that could be the cause of the problem
I've seen L - N voltages of 380v on sites where the neutral has lost it's earth reference

 

[MDJ]

Have you checked for a floating neutral, that could be the cause of the problem
I've seen L - N voltages of 380v on sites where the neutral has lost it's earth reference

yup, you say floating I say hung, same thing mate, looks like we are on the same page with this one

 

[MdubElec]

if the neutral is switched in the board or via a contactor for example and it fails you will put 230 volts to earth via the neutral and 400 volts across the 230 volt circuits and will burn out all electronic equipment. this can happen and is why you usually never switch the neutral in TPN systems where single phase circuits are connected, I have seen this several times in my career, it may not be the case, and I am only guessing in this case but get your sparky to check if the neutral is switched somewhere between the MCCB panel board to the distribution board. If on this occasion it is a hung neutral then there could be trouble ahead mate, I hope it is not this

I get that if a neutral was lost, the neutral bar would be a live part if it had one circuit connected to it from one phase...but if there were multiple circuits from all 3 phases going to the neutral bar would it not just dead short? And why would 400V be put across the single phase circuits?

 

[MDJ]

I get that if a neutral was lost, the neutral bar would be a live part if it had one circuit connected to it from one phase...but if there were multiple circuits from all 3 phases going to the neutral bar would it not just dead short? And why would 400V be put across the single phase circuits?

easy really mate, look at a three phase star, all 3 phases join together with the neutral connected to the centre point. now if the neutral hangs or floats as UNG rightly mentioned then instead of getting 230 volts to neutral from the phases you will get around 400 volts and the neutrals in the board will have a voltage of 230 volts to earth from them. this means all single phase circuits connected to the board which utilises a neutral will end up with 400 volts between the phase and neutral instead of 230 volts, all electronic equipment will burn out, I have seen 30ks worth of cookers in a school burn out when a DB controlled via a contactor with the coil connect to a stop circuit fail, the DB had a 4 pole switch and not a 3 pole, this has happened a lot hence why you should not switch a neutral on a 3 phase system with single phase circuits.

 

[MDJ]

Also the top guys will be on soon after they have had their dinner and I am sure one of them will give more info and maybe even a drawing up, of course this may not be the case but it sounds very much like it to me.

 

[MdubElec]

easy really mate, look at a three phase star, all 3 phases join together with the neutral connected to the centre point. now if the neutral hangs or floats as UNG rightly mentioned then instead of getting 230 volts to neutral from the phases you will get around 400 volts and the neutrals in the board will have a voltage of 230 volts to earth from them. this means all single phase circuits connected to the board which utilises a neutral will end up with 400 volts between the phase and neutral instead of 230 volts, all electronic equipment will burn out, I have seen 30ks worth of cookers in a school burn out when a DB controlled via a contactor with the coil connect to a stop circuit fail, the DB had a 4 pole switch and not a 3 pole, this has happened a lot hence why you should not switch a neutral on a 3 phase system with single phase circuits.

With the main switch for the DB off the voltage reads as normal, with it on, it reads as normal. With a circuit that only has sockets on it switched on the voltage reads normal. Once a circuit with a fixed load is switched on the voltage starts acting strange. This is info is from phone calls but I do trust the spark, maybe I need to get on site tomorrow to check it out.

 

[MDJ]

With the main switch for the DB off the voltage reads as normal, with it on, it reads as normal. With a circuit that only has sockets on it switched on the voltage reads normal. Once a circuit with a fixed load is switched on the voltage starts acting strange. This is info is from phone calls but I do trust the spark, maybe I need to get on site tomorrow to check it out.

I am only giving you a reason why it happened mate, I could be wrong and really hope I am. sockets are irrelevent, they are not sensitive electronic equipment, that said it may be something else, I am just suggesting what may have occured, good luck, hope it is something else fella.

 

[UNG]

With the main switch for the DB off the voltage reads as normal, with it on, it reads as normal.

Without any load the voltages will read normal as all the phases are balanced

With a circuit that only has sockets on it switched on the voltage reads normal.

With nothing plugged into any of the sockets the phases are still balanced

Once a circuit with a fixed load is switched on the voltage starts acting strange.

So now you have a load unless the loads are equal on all the phases the neutral point will shift (float or hang, take your pick), when this happens the phase - neutral voltages will vary between each of the phases and neutral

This is info is from phone calls but I do trust the spark,

The problem these days a lot of sparks are not up to speed with three phase faults and if you have never seen it before a floating neutral can cause some head scratching until you cotton on to what it happening

maybe I need to get on site tomorrow to check it out.

Depending what the site is and what loads it has I would be getting there pronto and isolating the installation before something gets cooked and causes a fire and there isn't a building to go to tomorrow