if you thought last nights question was bad

[gap30]

Just got home, installed a spur at a school today, spurred off a socket blah blah, did usual test and now i am filling out a meiwc on easycert, it was a c type 32a 61009, puts the Zs down as 0.92 and waheey its flagged up, max Zs for this device is 0.72

Do i just mention it on cert and thats it, i always thought if you go over your max Zs you either increase CSA, use RCBO (which it is) or install supp bonding

 

[Darkwood]

Take OSG guide next time and review your results before leaving it will be possible double check your results and not rely on a computer to flag the issue up, as well as weigh up any notes needed for the customer like supplementary bonding to the dist' - board, been a school id also be inclined to review any previous records if exist and see if the Zs has changed which would hint an issue that needs to be addressed either for the sub-mains supply or the circuit you added to, but as you say its got rcd protection so not a big issue, on a standard type b mcb it would be well within it allowance its just the pain that is some brands of rcbo dont do type b's

 

[gap30]

Its not 1.44

 

[Darkwood]

Sorry il self correct here... my advice may mislead, it has to according to table 41.3 comply to 0.72 for either 60898 or 61009-1... so with respect it is too high and should be flagged but where rightly mention it has rcd cover within the device but still believe you need to address the issue to reduce the zs value to comply, i myself am interested on others thoughts on this as its an issue i havent really thought about, ??

 

[gap30]

I have flagged it up on the cert, but mentioned it has rcd protection so is compliant

I will speak to school regarding it in the morning, i was subbying for another company that installs roller shutters

 

[spark 68]

My thoughts are that if it is only slightly over the max Zs, and is expected as a reasonable reading, within reason (ie. what you calculated), then an RCD or RCBO can 'mitigate' this scenario and still be within the regs.

What I personally dislike, and find unacceptable is where an RCD is used to mask a fault, or to bypass an unexpected high reading.

 

[StuSpiers]

chances are if you put it on an mcb for testing purposes and tested it on high loop (not no trip) it would pass anyway.

Just make sure to reinstate rcbo.

 

[Guest55]

Did you take a Zs reading off the existing socket before installing the new socket / spur ?
How long is the cable run youve installed ?
Could increasing the size of the new cable to 4mm or even 6mm reduced the ZS readings
to within permitted values ?
Lots to consider lol.

 

[gap30]

it was an existing ring i have added about 1m of 2.5 to it, its wired in 2.5 all looks pretty new to me, did not even give it a second thought to be honest

so used to 1.44

the end to end was pretty high 0.95, 0.95 and 1.60 i remember thinking thats one big --- ring but i did not count number of points as it does not ask for it

 

[Guest55]

My thoughts are that if it is only slightly over the max Zs, and is expected as a reasonable reading, within reason (ie. what you calculated), then an RCD or RCBO can 'mitigate' this scenario and still be within the regs.

What I personally dislike, and find unacceptable is where an RCD is used to mask a fault, or to bypass an unexpected high reading.

Agree with this ^^^
Rcds are no excuse for cr*p readings that should be dealt with at he design stage.

 

[Darkwood]

My thoughts are that if it is only slightly over the max Zs, and is expected as a reasonable reading, within reason (ie. what you calculated), then an RCD or RCBO can 'mitigate' this scenario and still be within the regs.

What I personally dislike, and find unacceptable is where an RCD is used to mask a fault, or to bypass an unexpected high reading.

Also to mention on the 'slighty over' reading is that OP should be flagging a reading if its above 0.8 x permitted zs and in this senerio- 0.58ohms and over may warrant a note or further investigation result dependant.

 

[StuSpiers]

yhea you have an r1+r2 of 0.64 calculated so that doesn't give much spare for your ze and still come under 0.72

 

[Darkwood]

Quick way around this is to fit 32(b) mcb and run through a seperate rcd in an enclosure, this then meets requirements but only do this if zs is a result to be expected in comparison to the dist' boards incoming zs, if the circuit itself is unusually high against the D.B.'s zs reading then it points to a connection issue at one of the termination points.

 

[telectrix]

1 poor connection at a S/O could be the problem. also internal resistance of the RCBO could contribute to a higher than expected reading. what was the Ze?

 

[gap30]

Also it was a sub mains with about 8 blanks missing, it had been tested in January and there was a massive list of remedials according to the caretaker, i just didnt give the reading (0.92) a second thought

God i feel stoopid now, as i said i have flagged it

Did not carry out Zdb or pfc - just tested the circuit itself