Quick Query To The Sparks

[securespark]

I went to a NO POWER job today. It was a PME supply and the neutral was open circuit and showing live. Earthed metalwork OTOH, was not showing live.

I isolated it and contacted the DNO on behalf of the customer.

They came out & disconnected the main earthing conductor from the cut-out & set up a genny, without an earth rod.

So, my questions.

Bearing in mind the neutral on the PME supply was open-circuit and the CU was isolated, was the DNO's engineer making the installation safer by disconnecting the earthing conductor?

The genny temporarily feeding the house had an RCD protected output. However, I could not see an earth rod connected to the installation. Was their temporary installation safe?

 

[oharaf]

I am suprised the genny was not supplied with an earth i.e. rod

Where / how was the genny connected to the house supply?

 

[securespark]

It was connected to the CU tails via a Henley.

 

[oharaf]

Right, so there really should be an earth rod!!!!

Any idea what has caused the complete loss of?

 

[securespark]

The supply to this house was from the adjoining semi: all the eng said was that the connections were down in next door's cut-out.

What makes me laugh is when these supplies are fed from properties the DNO has no access to.

My mate had power down. The DNO called. His supply came from next door.

They were in Africa.

For 4 months!!

The DNO offered a genny. He declined. Got them to dig up the street and give him a new independent supply!!

 

[Spark123]

Disconnecting the earthing conductor probably wouldn't matter that much as it is open circuit upstream.
They should however have connected the generator earth to the house in it's place.

 

[bernardgreen]

My mate had power down. The DNO called. His supply came from next door. They were in Africa.

Doesn't the DNO have a right of entry ( subject to police supervision and making good any damage ) to ensure necessary safety procedures were carried out ? Based on way leave agreements for the cables through the wall. That said the outcome of the new supply was the best result.

The genny temporarily feeding the house had an RCD protected output. However, I could not see an earth rod connected to the installation. Was their temporary installation safe ?

I asked that question when a genny was feeding a house. The reply was the the genny output was fully floating. The DNO engineer compared it to the floating earth free shaver supply in a bathroom being safe to use.

 

[securespark]

OK. But an appliance for use in a shaver socket is Class II. What about Class I appliances connected to this "floating" supply with no rod?

 

[bernardgreen]

If the genny's output is floating earth free then it is "" safe "" to touch either one of the terminal of the output as there is no route for a current to flow through the body and earth to the other terminal.

But if one terminal of the genny's windings was earthed then touching the other one would complete a circuit via earth and the body across the terminal voltage.

If the genny's winding was centre tapped and this was earthed then touching either terminal would create a path via earth and body across half the genn'y voltage.

 

[JohnD]

The genny temporarily feeding the house had an RCD protected output.

so if unearthed, the RCD is useless, right?

 

[Adam_151]

The genny temporarily feeding the house had an RCD protected output.

so if unearthed, the RCD is useless, right?

Not really, its effectivly an IT system, but a first fault would lead it it becomming like a TT system (and if the fault were 'live'->earth then the polarity would be round the wrong way to boot!)

RCD makes sense to me... if reducing an inconvience were a prime concern then an IT system would have an insulation monitor that would detect the first fault and flag it up before a second one occured and tripped the fault protection out.

... Thats my understanding anyway, I don't work on IT systems so I'm not that up on them

 

[Spark123]

RCD protecting an electrically "Separate" source is pretty pointless.
One fault would have to be upstream and one downstream of the RCD in order for it to work.
Also need to have double poled protective devices as at least in theory, it is possible to have a L-CPC fault in say a shower circuit which would remain undetected, then if there was a N-E fault on a lighting circuit the result being a dead short via the CPC of the shower circuit, CPC and neutral of the lighting circuit. The fault would have to take out the shower fuse to disconnect.

 

[securespark]

So, as I thought, then. A rod should have been installed...

 

[Spark123]

I think a rod should have been installed as the Means of Earthing, the neutral and protective conductor joined to it at the generator.

The L&N could then be run in as is, and the protective conductor joined to the MET in the house, making it a TN-S system.

 

[Steve]

OOI, what size generator do the DNO use for temp supplies like this, and where do they put them? Are they noisy?