Visual Inspection - try your luck

[NotHimAgain]

Visual inspection - try this DB - two views of the same DB - can you spot the most significant error .

 

[Steve]

no labelling, tape used as lockoff, are the blanks in the lid? If not thats a fault.

 

[Lectrician]

Had the same thing not so long ago.

N and E bars confused.

 

[NotHimAgain]

Lectrician has it .

This is a Dorman Smith DB in a large 'high class' installation carried out by one of the largest contractors in the country. What makes it worse is that it was installed about 6 years ago and other contractors have worked on it since

Of course the test results from the initial verification were detailed - perfect results from all of the tests .

 

[Lectrician]

Those boards have always annoyed me by having the N and E bars diagonally from each other!

I found it on an old MK Sentry TP board. Lucky with that one we could just remove the whole bar and swap the mountings over so the neutral was isolated from the case, and the earth bolted directly to it.

You have to be a little warry, as no doubt the outgoing earths are 'earthy' via parallel paths, meaning you have an N to E short. In a TN-S installation this can be a little nasty with high currents flowing.

Just the other day I worked on a site with TN-S, and a substantial supply. There was a generator feeding a single TP board via a changeover switch. Unfortunately the changeover switch did not switch the neutral (old school). The generators N-E link is allowing around 40amp to flow from the N to E. The guy servicing the generator had a scare when he was working on it and open circuited the joint.

They have priced up to change the contactors in the changover switch for 4 pole ones.

The closer the N to E short is to the origin of the installation, the worse this can be.

 

[electronicsuk]

Shame, as it looks like their put in a bit of time in keeping things tidy.

 

[Lectrician]

Shame, as it looks like their put in a bit of time in keeping things tidy.

Perhaps thats why when they had finished, and spotted their error, they ignored it

 

[NotHimAgain]

I discovered this on a Saturday afternoon during part of a rolling PIR programme I do for the site owner.

The installation is large (7 * 11kV / 400 V sub-stations and around 5000 final circuits).

The building has a large steel frame and the sub-station related to this DB is roof mounted about 70 metres away - so the 'danger' was not that great. However, I raided the facilities department stores and found enough cable and barrel crimps to extend the RHS top neutrals down to the correct position. I was under a 'permit to work' scheme so I could not open other DBs to see if there were any more. The facilities electricians found another 5 during the next week .

It should have been spotted when the lower RHS cpcs were connected to the bar as there is a bl**dy great black conductor just below them .

 

[Spark123]

The installation is large (7 * 11kV / 400 V sub-stations and around 5000 final circuits).

I'd have thought that a system like that would be more suited to BS88 fuselinks than MCBs, what are the fault levels?

 

[NotHimAgain]

No more than about 5 kA at the DB intake point. Each sub-stations has a pair of 1600 kVA transformers operating separately so your maximum fault level at the sub-station is under 40kA (assuming Zpu ~ 0.06 - which is fairly common for that size transformer).

Each distribution circuit is protected by a BS 88 at the sub-station panel - this provides back up protection for downstream mcbs at these fault levels by limiting let through energy.