Question about DNO changing supply characteristics

[thebigcheese]

Hi all, long time lurker here!
Was walking past Shepherds Bush green on Saturday evening to see a load of UK Power Network people digging up the pavement and an entire row of shops with no lights or power (!)

On Monday morning while they were carrying out repairs about 4 gennies had been set up with cables running into the various shops. Next to one of the gennies was a short unconnected cable with an earth rod attached to it. These gennies were plonked on the pavement and short of running the cable across the road the was no means installing the rod anywhere useful hence why it was just lying on the ground I guess.

I’ve got a couple of questions…
Assuming that the original connection to the supply was TNCS what precautions do the DNO have to take when temporally supplying a property with a TNS (or indeed unearthed TT) system? I mean would they have to calculate disconnection times for each property and only energise the supply if these were met?

Second question… assuming the supply from the gennie was TNS… what would the ELI be like? Would the fact that the cpc was bonded to the neutral guarantee a very low reading despite being separated from the actual mass of the earth?
Am very interested to hear you replies!

 

[Engineer54]

Hi all, long time lurker here!
Was walking past Shepherds Bush green on Saturday evening to see a load of UK Power Network people digging up the pavement and an entire row of shops with no lights or power (!)

On Monday morning while they were carrying out repairs about 4 gennies had been set up with cables running into the various shops. Next to one of the gennies was a short unconnected cable with an earth rod attached to it. These gennies were plonked on the pavement and short of running the cable across the road the was no means installing the rod anywhere useful hence why it was just lying on the ground I guess.

I’ve got a couple of questions…
Assuming that the original connection to the supply was TNCS what precautions do the DNO have to take when temporally supplying a property with a TNS (or indeed unearthed TT) system? I mean would they have to calculate disconnection times for each property and only energise the supply if these were met?

Second question… assuming the supply from the gennie was TNS… what would the ELI be like? Would the fact that the cpc was bonded to the neutral guarantee a very low reading despite being separated from the actual mass of the earth?
Am very interested to hear you replies!

Question 1 ....what precautions do you think the DNO have made ???
Question 2 .... Why do you think the N-E connection would still be in circuit when they are working on the shop supply lines. I don't think they would be that stupid!! lol!!

 

[mechelec]

I would suspect the genies had earth fault protection. There should have been an earth rod installed though. Maybe one at the load end?

 

[thebigcheese]

Q1, Well I'm not sure. I was under the impression that people working for the dno were not required to have 17th edition and so would be unable to make such calculations. I was wondering if there was a code of practice for such occurrences or whether as it's only temporary it doesn't really matter??

Q2, I was thinking more along the lines of the link within the gennie

 

[mechelec]

Q1 the DNO dont work to BS7671 althought the disconnection time for any part of their installation within a customers property does need to comply. Most work carried out by DNO empoyees is covered by codes of practice. This means that although many cannot make the calculations for themselves some else has and written it into a policy document.
Even though it is a temporary installation it has to be be correct as public safety is a priority.
Q2 most generators over 10KVA will have the neutral point referenced to earth and the chassis. This is usually acheived by as you say a link in the generator between neutral and earth.