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100Ma Type S Rcd


jimmybobbob

Question

jimmybobbob

Hi,

 

maybe I'm being thick, stupid, or plain ignorant.....but can anyone tell me what the maximum Zs permitted is for a 100mA Type S RCD.

 

Many thanks

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13 answers to this question

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Hi There

 

32A "B" curve, tripping current to trip the thing in the required time is 5 times its rated current so 5 x 32 = 160.

 

You got 230v so V/I = R [as Andy said] gives a Zs of 230 / 160 = 1.43 ohms. Bearing in mind that when you measure it with your meter you will need to be below 0.8 of that, so 1.14 ohms

 

You are obviously not going to be able to achieve this, so it is back to the RCD to provide ADS.

 

You could use the type "S" 100mA one to protect the supply cable as it is a distribution circuit, BUT from then on you will need "ordinary" ones to achieve the disconnection times for the final circuits to the sockets that feed the caravans or whatever, as a type "S" one will be too slow..

 

I am certainly not an expert at this, so check with the others if this sounds ok!!!!!

 

john..

 

what about Cmin too? ;)

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Andy™

R = V/I

 

S type or normal is irrelevant

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Hi,

 

maybe I'm being thick, stupid, or plain ignorant.....but can anyone tell me what the maximum Zs permitted is for a 100mA Type S RCD.

 

Many thanks

Depends what OCPD you are using.

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jimmybobbob

the supply cable coming in is protected by a 32A 60898, (part of a tncs system) then the consumer cable goes out on a TT system protected by another 32a 60898.....

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apprentice87

Ah, well....

 

Not as simple as all that....

 

It would be tempting to think 2300 ohms. [and i assume that you are using it as a front end RCD] BUT, and this is a VERY BIG BUT. further downstream you are almost certainly going to be fitting a 30mA rcd for shock protection purposes.

 

Now, it is tempting to think that the max Zs for one of them is 7666 ohms, but it is not, as you need to achieve a touch voltage of less than 50V, so now you need a max Zs of 1666 ohms, BUT once again it all depends, as has been stated, on what OCPD you are using, AND which is intended to be the means of achieving ADS, the OCPD, OR the RCD.....

 

john...

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jimmybobbob

Andy...I appreciate the simple math......just cant see it in bs7671 for type s

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apprentice87

What curve breaker though?? what about discrimination??

 

Anyway, assuming a "B" curve it would be 1.43 ohms so you really need a Zs of less than this.... However, in a situation where this is not achievable [like your TT] then it is permitted to use an RCD to achieve disconnection in the required time so you would be looking at 1666 ohms and preferably much much lower.... like about 100.......

 

john..

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jimmybobbob

yes there is 30mA rcd's downstream for ADS protection

 

I'm basically supplying 3 stations with plugs on for camping purposes. the 100mA rcd is for the fault protection on the cable and the 32a mcb is for overcurrent

 

 

 

Yes it is a type B

 

as for the discrimination I think that went out of the window a long time ago on this particular job.....it has been added on and changed so many times previous its hard to put any discrimination in on what disconnects first ie. there are 3 consumer units upstream before I get my power to play with

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Andy™

as I said, S type or normal is irrelevant, they are both the same

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apprentice87

As Andy said....

 

john....

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jimmybobbob

ok so for 100mA its 500 on paper (but we all know 200 or less is optimal) and so the correct disconnection time would be 0.2 as its a final circuit not a distribution circuit which permits 1....is that right?


What curve breaker though?? what about discrimination??

 

Anyway, assuming a "B" curve it would be 1.43 ohms so you really need a Zs of less than this.... However, in a situation where this is not achievable [like your TT] then it is permitted to use an RCD to achieve disconnection in the required time so you would be looking at 1666 ohms and preferably much much lower.... like about 100.......

 

john..

 

 

is that a typo john?

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apprentice87

Hi There

 

32A "B" curve, tripping current to trip the thing in the required time is 5 times its rated current so 5 x 32 = 160.

 

You got 230v so V/I = R [as Andy said] gives a Zs of 230 / 160 = 1.43 ohms. Bearing in mind that when you measure it with your meter you will need to be below 0.8 of that, so 1.14 ohms

 

You are obviously not going to be able to achieve this, so it is back to the RCD to provide ADS.

 

You could use the type "S" 100mA one to protect the supply cable as it is a distribution circuit, BUT from then on you will need "ordinary" ones to achieve the disconnection times for the final circuits to the sockets that feed the caravans or whatever, as a type "S" one will be too slow..

 

I am certainly not an expert at this, so check with the others if this sounds ok!!!!!

 

john..

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You really need to refer to Section 7 of BS7671 which will detail everything you need for caravan supplies.

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