pscc

fog · Sep 15, 2013

Hi can you refresh my memory!
I am downstream from an RCD, trying to obtain PSCC do not wish to use meter as will trip RCD, can someone remind me of the calculation to obtain PSCC, to complete PFC.
230v, 45.9 ohms on TT stake.
Many thanks

Sintra · Sep 15, 2013

No point using the 45.9 ohm value from the rod. What you need to do is carry out a loop test between L & N which would be a lot lower. Then use this figure to work out the PFC.

fog · Sep 15, 2013

Sintra said:
No point using the 45.9 ohm value from the rod. What you need to do is carry out a loop test between L & N which would be a lot lower. Then use this figure to work out the PFC.

Hi yes, the problem i have is i do not wish to knock out the RCD which is upstream of the sub board, would it be possible to calculate, the RCD is in main house which i dont have access to, and even if i did surely this test would not be possible as the RCD would trip out?

telectrix · Sep 15, 2013

dead test R1+Rn. then PSCC= 230V/( R1+Rn).

fog · Sep 15, 2013

Yes many thanks that makes sense, will use that method. Thankyou telectrix

Octopus · Sep 15, 2013

Can't you use the no trip setting?

telectrix · Sep 15, 2013

with most MFTs, the no-trip is 3 lead test and gives you the pefc. seems OP wants pscc, so that should be do-able using 2 leads. the RCD should not react as you are not connecting to E.

Sintra · Sep 15, 2013

telectrix said:
dead test R1+Rn. then PSCC= 230V/( R1+Rn).

fog said:
Yes many thanks that makes sense, will use that method. Thankyou telectrix

Surely to do this you still need access to the RCD to isolate.

telectrix · Sep 15, 2013

i'd trip it with MFT and let the silly arse that stopped access worry about resetting it.

fog · Sep 15, 2013

Sintra said:
Surely to do this you still need access to the RCD to isolate.

hi have access to the fused connection unit, that the supply comes from in utility room, but the lovely.... householder would not allow access into main house, must of thought i was an electrician who is also a burglar ho ho, we cant choose are customers, well we have a bit of lee-way. all done now using R1 + Rn method
230/(R1+Rn)
Thanks for all the help

SirKit Breaker · Sep 15, 2013

OHMS law

Geordie Spark · Sep 15, 2013

SirKit Breaker said:
OHMS law

Something to do with George Lazenby (remember him?) in the starring role wasn't it???

tidyboiler · Sep 15, 2013

Geordie Spark said:
Something to do with George Lazenby (remember him?) in the starring role wasn't it???

Not as good as Rog...........:94:

Geordie Spark · Sep 15, 2013

tidyboiler said:
Not as good as Rog...........:94:

No pet ... there's nothing like a good Roger is there?

telectrix · Sep 15, 2013

p'raps that's why he's called roger MORE.

ipf · Sep 15, 2013

Lazenby had a bloody big FRY

Geordie Spark · Sep 15, 2013

ipf said:
Lazenby had a bloody big FRY

That's good.

Tidyboiler enjoys a bloody big fry up first thing in the morning.

tidyboiler · Sep 15, 2013

Geordie Spark said:
No pet ... there's nothing like a good Roger is there?

As in "Roger the cabin boy".............. :ninja:

ipf · Sep 15, 2013

Aye, thought she'd be up for a bit of Turkish Delight with her sausage and scrambled.

Geordie Spark · Sep 15, 2013

tidyboiler said:
As in "Roger the cabin boy".............. :ninja:

Stoppitnow .... before you go any further. :38:

telectrix · Sep 15, 2013

awwww. just because it's your turn in the barrel.

ackbarthestar · Sep 15, 2013

telectrix said:
dead test R1+Rn. then PSCC= 230V/( R1+Rn).

I can't see how that will work, IMO, as the OP will need access to the front end of the cct. So its a guesstimate.

Since you'll need Zline ext. + R1 +Rn of the sub-main and as the OP doesn't have access to either then its going to be a calculated value based on assumptions about the supply cable type, size, length and the same for the sub-main cable.

We're not even sure whether the supply earthing arrangement is TN or TT, only that the far end supply is TT'd so we might struggle to get a reasonable supply voltage off and on load and calculate the resistance from the volt drop at the far end....

I'd just go for the 'drive-by' calculation if I was you.

that is:
Guess one + guess two + Variations in terminal resistances through various in line connections = 0.5 Ohms

This calculation is only for advanced users only - not recommended for the barn stormer

telectrix · Sep 15, 2013

as OP said he had access to switch fuse supplying his installation, i assumed he wanted the pscc for that point.

Jdsparky · Sep 15, 2013

My suggestion is do the live "no trip" test, or normal one and hope for the best,if it trips knock the door and say can you reset the Rcd please, then resort to other methods....how can you do all the other tests you may need to do at the origin of supply?

ackbarthestar · Sep 15, 2013

hi have access to the fused connection unit, that the supply comes from in utility room, but the lovely.... householder would not allow access into main house, must of thought i was an electrician who is also a burglar ho ho, we cant choose are customers, well we have a bit of lee-way. all done now using R1 + Rn method
230/(R1+Rn)

Ok, so R1+ Rn from the utility room to the outside installation which can be measured.
We still have R1 + Rn from the DB to the Sw Fuse, Zline ext plus a bit for terminal resistances.....

Just a few thoughts