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Topic Title: zs exeeded (just)
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Created On: 04 September 2016 03:52 PM
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 04 September 2016 03:52 PM
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Joined: 24 June 2016

hi guys i am happy to admit my ignorance if i get better but i do have pride so take it easy.

we have a circuit containg 3 small motors amongst other curent using equipment

on occasion they all start together and trip a b32 MCB

i have installed a c 32 RCBO to overcome this issue

of course the zs is now an issue but very close (worst at end of a spur)

the use and construction of the kitchen prevents me changing the circuits or cables at the moment .

i have over current protection
i now achieve much faster ADS than required under fault to earth condition

the supply to the dis board is a submain from another building
if i convert to TT i am fine but this offers less protection

can i argue as designer deviation from Bs7671 having insured = or additional safety

or am i over complicating ?

your thoughts cheers
 04 September 2016 04:07 PM
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Why are you saying the Zs is too high if an RCBO has been installed?

If you make a claim that the RCBO is providing fault protection, then I doubt if the Zs will be too high - well it won't be.

So you say you have overload protection, the RCBO will give you fault protection, so as long as the thermal constrains for the cable L-N are Ok, I don't think you have an issue.

TT or not, doesn't matter.
 04 September 2016 05:02 PM
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The RCBO will solve the Zs but may trip due to leakage current from the motors. When you say the Zs is close to maximum do you mean the 80% value of BS7671? You could measure r1r2 and add to the Zs of the DB feeding the circuit and using the On Site Guide correct for temperature and if bunched etc and you may find its ok. Also if you speak the CB manufacturer they may state a different Zs from BS7671

 04 September 2016 05:40 PM
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thanks guys

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