Measured Zs just over regs!!!

[rob69]

Can anyone give me the formula with ref to Appendix 14 of the regs??
Near the bottom of that page it says accurate calculation of Zs with
respect to load current and conductor temp.thanks

 

[Sintra]

It's all in appendix 14.

 

[Pete999]

What he said ^^^^^^^^

 

[rob69]

The formula near the top is not applicable to the state ment at the bottom which quotes the use of "Ib" in gaining a more accurate calculation of Zs or am I missing something.
I have two final circuits a ring 32A at 1.44 ohms and a cooker 40A at 1.16ohms both over regs limits!

 

[Engineer54]

New or existing circuits?? If existing, then the wonderful cover all RCD will suffice.... Well that's seems to be the way of things in the UK now (Not mine i might add)!! If New, meaning YOU have just installed the circuits, then you'll need to rectify your cock-up to bring them both into ADS compliance!!

 

[uksparks]

The NICEIC bloke picked me up on this last time I saw him, it was on a board I'd changed, existing installation, I'd measured the ze and added it to R1 + R2 to bunged the figure in the box for maz zs,and it was marginally higher, it was only because the cpc reading was too high, anyway, the zs reading at each socket was well under. He said I need to split the ring as it didn't comply.

I had changed every faceplate in the house and even rechecked them all to try to resolve the high cpc loop reading but didn't help.

He was saying that it would be ok on earth fault protection but not on overload or something then started going on about the time curves and tables in the back of the regs book.

 

[D Skelton]

New or existing circuits?? If existing, then the wonderful cover all RCD will suffice.... Well that's seems to be the way of things in the UK now (Not mine i might add)!! If New, meaning YOU have just installed the circuits, then you'll need to rectify your cock-up to bring them both into ADS compliance!!

Or reduce the OCPD rating? 32 down to 25 say.
Or supplementary bond?
Or change the type of protective device? C to B say.
Or change the sort of protective device? 3036 to 1361? Or 1361 to 60898?

There would be a whole host of ways I would go about remedying such an issue, including simply finding the source of high resistance and rectifying it, long before I would go down the route of installing a magical 'cure all problems' device such as an RCD!

That said, the wire by numbers guides don't cover such things, therefore they surely can't be allowed?!

 

[telectrix]

what's the Ze?

 

[ackbarthestar]

May I suggest that if you have checked the cpc and its still the weakest (highest resistance) then the simplest method to reduce the resistance value would be to cross bond to another circuit, such as the cooker. There are other ways, but most require an electrical upheaval...

 

[TJ Anderson]

If you have RCD protecting the circuit take 3-lead Zs at incoming and then outgoing terminals of the RCD. You may be surprised what it is adding to your readings. I always record on EIC or MW what impedance the RCD has added to the circuit.

 

[markc123]

The formula near the top is not applicable to the state ment at the bottom which quotes the use of "Ib" in gaining a more accurate calculation of Zs or am I missing something.
I have two final circuits a ring 32A at 1.44 ohms and a cooker 40A at 1.16ohms both over regs limits!

Im presuming this is domestic, so we can safely say their 60898's with it been a 32... Now, what equipment is requiring you to use a C or D type MCB on the ring main? Cause you can't be using a B type, as it complies ... JUST.

Granted the 40A is over by 0.01 ... mr.niceic might be fussy, so follow mr.skeltons advice.

Me thinks you're looking at GN3 for your values as opposed to the regs though! And as we all know the GN3 results are adjusted as the regs are at normal operating temperature.

 

[Risteard]

Given that he is inspecting and not designing he needs to apply temperature correction for unloaded circuits surely.

 

[sparkdog]

Try wiggling the test lead plugs.

 

[telectrix]

be OK when the byb comes out. if the readings are over the regs now, put the new one on top. it'll be heavier, thus making the readings lower.