RCD Not tripping?

[gcx51]

I have been asked to convert a mess room supply which is curently a TN-C-S to a TT.

Before I disconnected the supply and converted it i had the followng readings.

Ze = 0.21 ohms
PFC = 1.01kA
x1 = 28.1 (30mA)
x5 = 8.9 (30mA)


I am no longer using the earth supplied via the swa and have fitted a new earth rod and have taken following readings

Ra = 69.2 ohms
PFC = 2kA

The problem I'm having now is the 30mA rcd will not trip on any test?

The RCD worked fine before i converted it over to the new earthing system. Any ideas why it does not work now?

Cheers.
Rob

 

[nozspark]

Why has your PFC gone up??
it should be the same

Have you disconected the outgoing side of the RCD and tested the RCD in isolation?

If you have a N-E fault you will find that the RCD might not trip as the N resistance is much lower than the Earth rods resistance.

 

[gcx51]

Why would the PFC remain the same? I'm no longer using the earth/neutral supply, just using the neutral.

When I test between L & E the pfc is 2kA
When I test between L & N the pfc is 1.01kA

How can I have a N E fault?

This ones baffled me? I have carried out insulation resistance test which are are all ok and over >299. on all outgoing final circuits

I will try testing the RCD in isolation tomorrow on it's own.

any other thoughts?

Cheers for your help mate!

 

[Spark123]

PEFC of 2kA with a TT with an Ra of 69ohms is smelling of a parallel path.
You should have had a similar PEFC with TN-CS supply, probably higher.

 

[nozspark]

Why would the PFC remain the same? I'm no longer using the earth/neutral supply, just using the neutral.

When I test between L & E the pfc is 2kA
When I test between L & N the pfc is 1.01kA

Right, your L-N resistance is constant for both supply types, and lets say for a TNC-S supply is 0.35ohms at source. Now with a TNC-S supply the L-N and L-E resistance is the same. If you now convert the TNC-S to a TT your L-N resistance will stay the same and the L-E resistance will increase. This increase in resistance will reduce the L-E PFC, however your L-N PSC will stay the same. and as PFFC is the higher of the two it will stay the same.

How can I have a N E fault?

This ones baffled me? I have carried out insulation resistance test which are are all ok and over >299.

I will try testing the RCD in isolation tomorrow on it's own.

Cheers

All I can say is that this is sometimes the cause of a RCD not triping in a TT installation, check both the circuits from the CU and the submain.

 

[gcx51]

Thanks guys for your advice, I will isolate the rcd completely tomo and see what result I get. Maybe the pfc is not 2kA and I do have a Parallel earth.

Cheers

Rob

 

[ban-all-sheds]

I have been asked to convert a TN-C-S to a TT.

Why?

 

[Spark123]

I've only done a conversion that way around once, was for caravan site pitches.

 

[JohnD]

maybe my imagination ran away with me, but I was thinking "mess room" to be a site hut or portacabin kind of thing. Probably metal, with water services, so I was thinking, like a caravan, not to be TNC-S.

 

[gcx51]

maybe my imagination ran away with me, but I was thinking "mess room" to be a site hut or portacabin kind of thing. Probably metal, with water services, so I was thinking, like a caravan, not to be TNC-S.

Correct, it's a mess room at a cemertry. local council have requested this be changed after a recent pir.

The problem with the rcd not tripping turned out to be the rcd itself, i removed it and replaced it and he presto problem solved.

There was a parallel earth through the board fixings touching a metal part of the structure however the rcd still did not work in isolation.

With the new rcd fitted and the board back together the rcd is opperating correctly.

Cheers fellas!

 

[Spark123]

Don't you need to MPB the structural metalwork?

 

[Lectrician]

Yes. To the T side. The TN-C-S gland should ideally be terminated into a plastic enclosure outside the building to kep it segregated from the chassis.

It sounds like you have a galv box attached to the cabin, and have simply removed the earth from th CU? This is not good enough.

 

[nozspark]

I was starting to think the same thing (not dropped the sheath of the TNC-S supply)!!

Glad you managed to sort it though!!