quick test on a garage

[chris gray]

its a sub board fed from the house, how many sockets are there on the circuit?
the comment i made about the 250v test was as hed said hed done it n-e
'l-n at 250v for proving purposes is valid'

 

[WayneL]

Some dodgy replies to this thread, chaps - you all been on the cider??

hi its tn-s in the house, no the cables were not discotted at the garage cu didnt realise you had to do that, what are your thoughts on the R1+R2 thanks again

Tnc-s supply 6mm armour out to feed main board in garage

It's one or the other????

you did disconnect the cables for the test didn't you?

An IR Test - Why

If you tested the distribution board/installation as a whole then you need to disconnect the neutral/earth link for a TN system.

What neutral/earth link?

The live neutral fault could be due to the main switch not being open. If this is the case, then you would be seeing the neutral-earth link the PME head if it's TN-C-S, or back at the star point of the transformer if it's TN-S.

On an IR test?

thanks for all the replys so basically what i should have done was disconnected the ring from the board and then done the insulation tests?

No! No! No!

as for the insulation resistance, if you are testing 1 circuit only, you must disconnect it, (L,N+E) and of course disconnect/unplug any appliances and remove any bulbs etc,

Really?? GN3 page.....?

Oh dear - if he wasn't confused before, he will be now

 

[Risteard]

What neutral/earth link?

Neutral and earth are connected whether at the cutout in the consumer's installation or at the earthed neutral of the supply company's windings.

 

[WayneL]

Neutral and earth are connected whether at the cutout in the consumer's installation or at the earthed neutral of the supply company's windings.

Has no bearing whatsoever on your IR test as the main switch should be open, anyway, isolating the neutral.

It only matters in a three phase board, where the three phases are isolated, but the neutral isn't.

 

[Johno]

whisky actually, what don't you agree with

 

[WayneL]

whisky actually, what don't you agree with

Disconnecting conductors at CU for IR tests - & why?

 

[Johno]

because if they were still connected (depending on a few things) it would make a difference to the readings

 

[Risteard]

Has no bearing whatsoever on your IR test as the main switch should be open, anyway, isolating the neutral.

It only matters in a three phase board, where the three phases are isolated, but the neutral isn't.

Indeed. For some reason I was thinking of a three phase board which clearly isn't relevant to this situation. You're right - I must have been on the cider!

 

[WayneL]

because if they were still connected (depending on a few things) it would make a difference to the readings

I don't understand how it would make a difference to any readings, if the conductors are connected to their relevant bars and the mcb - you will have to be more specific.
- obviously the main switch is open at this time or you shouldn't be poking around in the CU anyway

 

[Johno]

who said he had opened the main switch, he might have just turned it off in the house

 

[WayneL]

who said he had opened the main switch, he might have just turned it off in the house

That's a very good point - I wouldn't have, but he might have.

In that case he needs to open the garage CU main switch before IR testing the garage installation - in case of any links neutral-earth back at the house.

And apologies Risteard, in a round about way, you also had a point

 

[Risteard]

And apologies Risteard, in a round about way, you also had a point

Maybe that's what I was thinking. I can't even remember anymore! So not sure whether credit is due or not...

 

[pevvers]

Has no bearing whatsoever on your IR test as the main switch should be open, anyway, isolating the neutral.

Hence why I said to check it...

Stranger things have happened, maybe he's just turned off the MCB feeding the garage board from the main board, and not the switch at the board he is testing, therefore he could be seeing the neutral-link at either the TN-C-S cutout, or the star point of the transformer.

Sometimes it can be as simple as that!

EDIT - just seen that someone else has brought up that point, but it's been known to happen, and could be the easiest cause for the neutral- earth short.

 

[WayneL]

You're probably right, pevvers - he didn't seem to clued up on testing....... with having to ask what the figure of 8 test was and not being sure what the earthing system is

 

[pevvers]

What do you mean by figure of 8 test pevvers?

This test is where (at the board, and dead) you interconnect leg 1 line of the ring to leg 2 neutral of the ring, and then leg 1 neutral to leg 2 line. After this you go to all the sockets and test between line and neutral pins on the socket outlets. The result at each socket should be approximately (r1+rn)/4.

Then you do the same but with leg 1 cpc to leg 2 line, and leg 1 line to leg 2 cpc. Then again go around all sockets testing between the earth pin and line pin. This result at each socket should be approximately (r1+r2)/4 which is where you get your calculation from.

If there are any spurs on the ring, this will show as a higher reading than the rest of the sockets, which would be able to be proved by taking the socket with the higher reading off. If there are two sets of cables in there, then the chances are you will have a spur off a spur, and remedial work will be required. This is an extremely useful test, as it shows spurs, polarity and a real R1+R2 reading all whilst dead...

P.S. Be aware that you should take the reading from the same side of each socket, as some cheaper sockets give incorrect (spurious ) readings on the two different outlets...

 

[jimmylaud]

thanks to everyone has helped alot i thought i knew what figure of 8 test was but was just checking, was a mistake saying tnc-s its defo tn-s, new to the testing so was just making sure i got everything done right

 

[WayneL]

Thought I'd thank you as well - that's quite a good description
.
.
.
And Jimmylaud, don't worry, it takes a bit to get your head around everything - I'm always stopping to think and double check things.
Even some 20-year sparkies have trouble with I&T.
The most important thing is to make sure you work safely

 

[pevvers]

And always better to ask than to do it wrong and wished you had asked....

I'm terrible for asking questions, but that's the reason I know some answers, cos it's questions I've asked before!!