Quiz Time :D

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DonkeyDong

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Time to play dumb and see how many different answers we get ;)

DB1 fitted with an RCD as main isolator (30mA), MCB from here feeds sub main to DB2 (which none of this can be changed)

DB2 has sockets and lights fitted on various breakers.

You have only worked on DB2 fitting these lights/sockets

Questions...

1) Do you record the RCD disconnection times for each circuit you've worked on (bearing in mind there is no RCD on DB2) ? (if required by the EIC)

2) The Zs is too high on one of your circuits in DB2 - Do you care as there is an RCD upstream ?

3) DB1 MCB is 20A Type B, DB2 MCB is 16A Type C, without comparing the charastaristic/graphs in the brown (now red) book - would this cause a potential issue with discrimination with overcurrent ?

Its Friday night...let the fun begin! :D

D.

 

Nicky Tesla

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1. I would say yes due to you having sockets on db2, but i would do the rcd test anyway because it is supplied by it. If it fails, and the rcd is not required on db2, the findings can be noted on any cert for your work on db2. In the 17th any unprotected cables in walls etc req rcd protection, if not in a supervised location, so this needs to be considered.

2.If this is a TNC-s or TN-S system, the Zs is required to be low enough to comply with the mcb requirements. Using the type C requires a lower Zs.

3.The 20A type B would trip instantaneously at 5x20A=100A and the 16A type C would trip at 10x16=160A, so this shows that the 20A MCB would trip first.

Also is there only one MCB on DB2?

 

DonkeyDong

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Howdy,

Glad to see someone replied.... 6 people have checked it though - Obviously they aren;t clever enough ;)

1.) Assume everything is either surface mounted or SWA

2.) But doesn't the RCD cover/cater for times when Zs is too high (ie on a TT). This system in question is TNCS

3.) DB2 has several MCBs, 3Ph, 1ph mixture of ratings.

D.

 

M107

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1) I'd say yes, just because it's the mainswitch on DB1 makes no difference.

2)So long as it comes in under the max given in table 41.5 all is ok I'd say.

3)Probably

 

DonkeyDong

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Oh dear...

Looks like we're turning into Screwfix - SL I think you might fit in better over there ;)

D.

 
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