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

 

[MdubElec]

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

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

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

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

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

We had the DNO out as we were sure it was a neutral fault at the time, they came out and said their supply was healthy which led us down the rabbit hole of thinking along the wrong lines i.e voltage spike etc.

How do we do our own checks on the incoming supply to confirm the fault? It looks like it was originally TNS but an earth has been linked to the neutral of the cut out making it PME. If all voltage reads correctly does it require more in depth dead testing? In which case the incoming supply would need to be isolated by the DNO, which would also mean they would need to come and check it again themselves?

 

[westward10]

For a TN system the neutral should have a reference to earth and in normal circumstances you should have 0v between them. Isolate the distribution board, prove it is not live and carry out a continuity test between neutral and earth. This can only be done if the neutral is not disconnected by isolation of the board.

 

[James]

For those of us that have seen a failed N in a 3 phase system as you are describing, we all know that a failure of the N is the most likely and almost certain cause.

you need to get on site and check for N continuity, I would start at the main board, do your dead tests and if all ok and you have a 400v heating element then put it between any phase and N.
then re check you have 0v between N and E.

keep moving along the distribution path and eventually you will find the fault. it is likely to be the blue wire with blackened insulation and tarnished looking copper.

 

[UNG]

We had the DNO out as we were sure it was a neutral fault at the time, they came out and said their supply was healthy which led us down the rabbit hole of thinking along the wrong lines i.e voltage spike etc.

How do we do our own checks on the incoming supply to confirm the fault? It looks like it was originally TNS but an earth has been linked to the neutral of the cut out making it PME. If all voltage reads correctly does it require more in depth dead testing? In which case the incoming supply would need to be isolated by the DNO, which would also mean they would need to come and check it again themselves?

With little or no information on the electrical installation you have on site or what other electricity users there are in the locality it is not easy to make suggestions
As an example about 10 years ago I got called to a Day Nursery that had an electrical problem, the installation was on a single phase supply from the DNO tested the incoming voltage at 180 - 190v so I knocked at the property next door and asked if it was ok to test their incoming voltage and found a voltage about 260v so I repeated the process with couple more properties and the common thing was the voltages were different were fluctuating and would not give a reasonably steady reading for more than a second or two and varied by more than 25 - 30v when observed for a minute or two. This was about 9:30 in the morning the building I was called to was at the entrance to a close with a number retirement properties the voltage fluctuation was more than likely caused by the loads of the kettles and toasters being used for breakfast, reported it to the DNO and it was found to be a failing joint in the pavement on the main road passing the close
Some good advice has been posted by a number of members on this thread but in a lot of cases it relies on the person on the ground understanding how three phase systems are constructed and work and applying that with a methodical fault finding approach is the way forward the fact that you called the DNO in to check the supply suggests that the guy on site doesn't have enough three phase knowledge as a floating neutral issue can normally be found and the source identified before getting the DNO involved

 

[MDJ]

We had the DNO out as we were sure it was a neutral fault at the time, they came out and said their supply was healthy which led us down the rabbit hole of thinking along the wrong lines i.e voltage spike etc.

How do we do our own checks on the incoming supply to confirm the fault? It looks like it was originally TNS but an earth has been linked to the neutral of the cut out making it PME. If all voltage reads correctly does it require more in depth dead testing? In which case the incoming supply would need to be isolated by the DNO, which would also mean they would need to come and check it again themselves?

With absolute respect to you fella, your Electrician should be able to understand how to test between phases and phases to earth and phases to neutral to determine if anything is amiss. The DNO supply is without doubt fine, the problem is your end, it is school boy stuff to test between cores, TNS and PME are types of earthing arrangement and nothing to do with a floating neutral which we believe may be the issue.

 

[MdubElec]

No one likes not know the ending to a saga so here goes.

It turns out it was a lost neutral from the DNO side, which we originally suspected and called the DNO out for. They did earth loop tests on their cables but no dead test continuity, so when we returned in the morning we tested their side and we found the fault between the cut out and the meter.

We had been led down the wrong rabbit hole due to the DNO falsely confirming that the neutral was there, we originally tested our side but decided to test their side after we had called them out once already.

Some damage to a few PIR's, the roller shutter door controls, AC stats and a few other little bits but not as catastrophic as it could have been.

Thanks for the help guys.

 

[MdubElec]

With absolute respect to you fella, your Electrician should be able to understand how to test between phases and phases to earth and phases to neutral to determine if anything is amiss. The DNO supply is without doubt fine, the problem is your end, it is school boy stuff to test between cores, TNS and PME are types of earthing arrangement and nothing to do with a floating neutral which we believe may be the issue.

I didn't explain myself properly, the reason I mentioned the PME was because the DNO engineer did loop readings at the meter to prove the neutral was healthy, but because of the PME he was getting the loop reading from the earth when testing the neutral.

 

[KungFuTurkeyLeg]

Glad to see a happy ending.

A strong sneeze would knacker a PIR, they seem to be sensitive to everything. Small single phase motors rarely survive a 400v blast either, but then neither would I