95 volts in an Earthing Conductor

[Greg:]

To make things worse the voltage is entering a shared Main Earthing Terminal from a separate premises with occupiers that don't seem to be that bothered about the situation. Things don't get better. In the section of the building which doesn't contribute to the voltage to the MET they've recently installed an electric shower. Its on a circuit with its own RCBO. Clearly the resi current part of the RCBO only supplies protection from earth currents generated within the equipment and not from currents supplied from external sources.

I got called to the building by the shower owning and guitar playing resident upstairs because of an apparent earth feedback problem on his electric guitar. So I checked the sockets and they all seemed to be be in order. And then, after checking there was no one downstairs and giving an instruction not to touch electric equipment or fixed items made of metal I started disconnecting earthing conductors.

After finding the 95 volts through testing I was intrigued to see the result when bringing the earthing conductor into contact with the Earthing Terminal. Sparks flew.

OK, a bit about the building. The ground (interesting term) floor is industrial. I guess the upstairs floor used to contain the offices related to the works. Supply goes straight upstairs and is now split to two meters (recently installed) and two visible DBs. There's also a large lever operated isolator connected to downstairs power. More on that later.

When the people downstairs returned they were good enough to let me take a quick look in the premises and after climbing my way over to a downstairs DB, which gave every appearance of being in operation and which the occupiers claimed was used to isolate non specified items of their equipment, I couldn't help but notice that the earthing conductor to this board had been cut and was hanging lose in the air.

A discussion with the people downstairs was called for. I received an explanation that all the problems were caused by the supplier and that they had tried many times to get independent supplies worked out for both floors of the building. I explained that this had nothing to do with the earth fault and that even though the Network Operator would supply a service head and meter there would be local responsibility to connect distribution and further explained that if for any reason there was a disconnection to earth and if someone was injured as a result then responsibility would lie with the downstairs property ... which, apparently, was not a problem. They are insured.

Any thoughts about this situation would be appreciated including in relation to switching off downstairs power when taking a shower. Not knowing what equipment they use downstairs I'm thinking that the people downstairs may need to be notified when power was about to go down and this complication is added to by the ~25 meter round trip that would need to be taken between the shower and the DB.

Edit: I forgot to say that the fault with the guitar turned out to be an unrelated problem with the pedal. It was only due to this mistake that I got the call out.

 

[Kamikaze]

Personally, I would ditch the voltage tester and logically using an insulation resistance tester track down the fault.

 

[snowhead]

Where was the 95v ?
What's the earthing at the meters, single MET? Earth arrangement?
Are the properties owner occupied?
Where does the cut earth originate?

Switching off downstairs when showering,, complete madness.
If there is a fault everthing should be turned off NOW, until fault found.

 

[guest119]

Did you check which direction the voltage is going, into you or exported?
Did you do a clamp test for current? This would indicate direction of flow.

 

[plugsandsparks]

You have, me thinks, an actual dangerous installation now and all should be done to determine how 95V is getting to the met. 95V is a nice voltage you get sometimes when earth is either used as a neutral or a fault on a piece of equipment. You need, i think to start to break down the installation until you identify which part of the installation is generating this voltage.

 

[Greg:]

You have, me thinks, an actual dangerous installation now.

Certainly and for now I will advise the folk upstairs of options of shutting down the downstairs electric until the problem has been dealt with.

Thanks all

 

[Greg:]

Where was the 95v ?
What's the earthing at the meters, single MET? Earth arrangement?
Are the properties owner occupied?
Where does the cut earth originate?

Switching off downstairs when showering,, complete madness.
If there is a fault everthing should be turned off NOW, until fault found.

Between the earthing conductor connected to the installation downstairs.

Upstairs is rented

Downstairs at garage style DB. I'm thinking that this is a danger in its own right and informed the occupiers of this so I could return to concerns of the earthing voltage.

- - - Updated - - -

Personally, I would ditch the voltage tester and logically using an insulation resistance tester track down the fault.

which I would do if I had immediate access to the downstairs property.

 

[R G]

What a dangerous situation ;
What you are experiencing when you re connect the cpc in the domestic premises is the fault current from the other unit back tracking in to the domestic unit and quite possibly down through the bonding . Disconnect other unit or connect the earh wire in their

 

[guest119]

What a dangerous situation ;
What you are experiencing when you re connect the cpc in the domestic premises is the fault current from the other unit back tracking in to the domestic unit and quite possibly down through the bonding . Disconnect other unit or connect the earh wire in their

Which is why I was asking which way it is going.
A spark when you pot two wires together doesnt tell you much.

 

[R G]

Agreed

 

[Greg:]

Which is why I was asking which way it is going.
A spark when you pot two wires together doesn't tell you much.

After my temporary disconnect of the earthing to the premises downstairs and the MET I checked for current between the MET and earth and there was none. I also found that the voltage in the earthing disappeared when I disconnected supply to downstairs and reasoned that the problem was with the electrics in that premises.

The problem would have similarities to a situation in which an adjacent property experienced high earth leakage. The significant difference being that the earth path for the premises downstairs goes through a shared MET.

Sorry I should have replied to your question earlier.

 

[HandySparks]

Maybe I've missed these points, but:
1) What is the earthing system?
2) Between which two points were you measuring this 95V, and under what circumstances?
3) What instrument were you using to measure the voltage? (ie high impedance / low impedance)
4) How did you check for current in the earthing conductor (between MET and means of earthing)?

It might also be useful to know:
a) What the Ze at the incoming supply / MET is.
b) What the current in the conductor that you disconnected was when you re-connected it.

 

[Greg:]

Maybe I've missed these points, but:
1) What is the earthing system? TNCS
2) Between which two points were you measuring this 95V, and under what circumstance? The earthing conductor from the isolator that supplies a feed for the neighbouring/ downstairs premises
3) What instrument were you using to measure the voltage? (ie high impedance / low impedance) high
4) How did you check for current in the earthing conductor (between MET and means of earthing)? across a variety of connections to the MET with volt meter

It might also be useful to know:
a) What the Ze at the incoming supply / MET is. a reasonably low value
b) What the current in the conductor that you disconnected was when you re-connected it. The earthing conductor from the neighbouring premises was reconnected to Earth via the MET (the only available route to earth) and unless something had changed within the neighbouring premises I presume the voltage to earth remained the same

responses inserted

 

[HandySparks]

When you're measuring stuff, you need to be clear what you're measuring. Voltage has to be between two points (you can't have a voltage in isolation, it has to be relative to some other point), and current has to be measured with an ammeter (or a voltmeter and a known resistance).

Maybe I've missed these points, but:
1) What is the earthing system? TNCS
Good.

2) Between which two points were you measuring this 95V, and under what circumstance? The earthing conductor from the isolator that supplies a feed for the neighbouring/ downstairs premises
OK, that's one point, but what was your reference point?

3) What instrument were you using to measure the voltage? (ie high impedance / low impedance) high
Which means that it could be just an induced voltage or via a very high impedance line to earth fault.

4) How did you check for current in the earthing conductor (between MET and means of earthing)? across a variety of connections to the MET with volt meter
So you're not measuring current, but voltage?

It might also be useful to know:
a) What the Ze at the incoming supply / MET is. a reasonably low value
Did you measure this? Any idea what the actual value was?

b) What the current in the conductor that you disconnected was when you re-connected it. The earthing conductor from the neighbouring premises was reconnected to Earth via the MET (the only available route to earth) and unless something had changed within the neighbouring premises I presume the voltage to earth remained the same


You need to invest in a reasonably sensitive clamp meter and actually measure currents. At present we're just guessing about what's happening.

Or maybe some insulation resistance testing on the two installations. I'd start with a global L+N to E test at the boards and go from there.

 

[Greg:]

Thanks handy for following this through.

2) Between which two points were you measuring this 95V, and under what circumstance? The earthing conductor from the isolator that supplies a feed for the neighbouring/ downstairs premises
OK, that's one point, but what was your reference point?

Sorry, this had been mentioned earlier but wasn't repeated. Between the conductor and the MET.

4) How did you check for current in the earthing conductor (between MET and means of earthing)? across a variety of connections to the MET with volt meter
So you're not measuring current, but voltage?

My view is that the potential for current flow was checked but not measured. Loop impedance tests checked continuity to earth while the voltage test checked the potential for the flow of a level of current which would clearly provide a potential for harm should further faults develop.

a) What the Ze at the incoming supply / MET is. a reasonably low value
Did you measure this? Any idea what the actual value was?

One of the first things I did after at this call out was to check Zs in an earth path test at sockets (sorry for the simple language but I've advised the client to view this thread to help them come to decision as to what to do with the downstairs feed). The main headline at that stage was that there was an Earth path but I seem to remember results that fluctuated around .63 ohms.

Or maybe some insulation resistance testing on the two installations. I'd start with a global L+N to E test at the boards and go from there.

I know that the neighbouring installation has IT equipment plugged in so without gaining contact with them i won't be able to get much meaning from insulation resistance testing.