UK Voltage between gas bond to neutral

[Kyleb2210]

So up until this morning I thought I had seen most faults in an electrical installation, but this one today has me stumped and I’m not sure if I’m being stupid and the issue is staring me right in the face! long story short we went to a domestic property with a TNCS to install a supply to a summerhouse, fed via a DP mcb in The Cu and a small Cu in the summerhouse feeding lights and sockets, all dead tests fine. Went to power up and we got 240v L to N, 240v L to E but 120v N to E just on the summerhouse circuit now I would of thought this would of taken the rcd out back in the house and the summerhouse Cu, bamboozled I started looking for a back feed as the IR test was clear through process of elimination I removed the bond to the gas from the Cu, and the fault went away, tested from the gas bond to neutral and got 120v! Disconnected the bond from the gas pipe outside and reconnected and the fault miraculously went away! Any explanation?

 

[Vortigern]

Someone crossed over the neutral and earth at some point in the circuit perhaps?

 

[Lister1987]

Check a neighbouring property, if present there too then I would call DNO reporting fault code A14 (polarity identified as incorrect at DB equipment) and they get someone out to resolve sharpish.

Reporting as per MOCOPA

When reporting a Category A (emergency) situation, you must always identify yourself as a Meter Operator and that you want to report a Category A (emergency) defect.

You should be asked to provide the following information:
• Your full name;
• Your contact telephone number;
• The Customer’s name and contact number;
• The Code of the most serious defect being reported;
• Any secondary defects and relevant information;
• The status of the supply;
• Location of defect, i.e. address including postcode;
• Meter Point Administration Number (MPAN);
• Who you are working for; • The registered Supplier for the property;
• Whether the work is associated with the installation of a Smart Meter;
• Where further evidence, such as photographs may assist the DB, you may be requested to provide such support. When you report an issue by telephone you will be provided with a unique job reference number by the DB Emergency Contact Centre on the same call.



ETA; obviously identify as an Electrician, as for what number to call it in on I would go for 105 which should connect to the area's DNO

 

[Wilko]

My first thought is - the new circuit does not have a solid N and so was sitting around 1/2 line volts. Then by the time you’ve jiggled the conductors around testing it’s found a spot and the V looks normal. If you have added a DP switch in the circuit, I’d start there as it could be faulty or loose N perhaps.

 

[pc1966]

Went to power up and we got 240v L to N, 240v L to E but 120v N to E

If they were all measured at the same place (summerhouse CU?) they don't add up.

Do you see the same at the main house CU?

Either there is a big phase shift (i.e. combination of 2 of 3 phases somewhere feeding this) or your meter is reading some 'phantom' component due to a floating cable and slight leakage.

The behaviour changing on removing/replacing the gas bond kind of suggests a poor connection somewhere that is being disturbed. Do you have a long enough wander lead to check continuity from house CU to summerhouse?

 

[Lucien Nunes]

The 120V measurement is a pretty clear indication that something was floating i.e. disconnected from its source; the question is what. As PC1966 says the initial set of three readings doesn't add up unless you factor in some weird stuff that I would tend to discount at this stage. If the voltage at appliances in the house is normal and there is no history of flickering lights etc, the neutral is probably OK, in which case a high-resistance CPC seems likely.

Beware reading too much into voltage readings when you have a suspect connection. The abnormal voltage tells you that there is a problem but not much more than that. There are some inferences one might make from the actual voltage (120V here) not yet enough information to make them admissible as evidence. The investigation has to progress by checking resistance and impedance.

 

[snowhead]

Was the 120v there with the supply to the house Off at the incomer, suggesting the 120v is from outside the property or phantom?
Or was it only there with the supply On?