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.

 

[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