RCD for Fault Protection

We had a situation at work where a customer had a FCU fitted in the back garden after he had a house renovation. He planned to have power to his shed down the bottom of the garden. 
 

He took the ZS from the FCU and it was 1.08 ohms. This means that adding 25m of cable would take it over the 80% of the 32A MCB. 
 

The customer wants to add 2 sockets and a light that would not exceed 13amps. The chap I was with said it’s not ideal but power could be put to the shed from the FCU as the RCD would cover it for fault protection.

I understand this is true but if we know it won’t meet ZS of the MCB should we plan to be reliant on the fault protection on an RCD ? 
 

The important question question is the OCPD the 13A fuse or the 32A MCB?

Murdoch said:

The important question question is the OCPD the 13A fuse or the 32A MCB?

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that’s a fair point this would give a bit more 

leeway. 
 

I assume that when the power is in the shed when fused down for the lights to a 3A fuse then using the 1362 would give more leeway for the lights. 
 

Thanks for flagging this up. But so I fully understand for future if the 80% Zs Is breached of the OCPD I guess it would be bad design to rely on the RCD as fault protection ? But if you in a position where you adding to an existing install and it can’t be helped then using RCD for fault protection is Ok. I know it’s probably a bad design if starting from scratch. 

You can't just add a RCD to get your allowed Zs increased (for fault protection) as you still have to ensure that your (R1+Rn) is low enough so that you have adequate overcurrent protection too

Traineeboy said:

that’s a fair point this would give a bit more 

leeway. 
 

I assume that when the power is in the shed when fused down for the lights to a 3A fuse then using the 1362 would give more leeway for the lights. 
 

Thanks for flagging this up. But so I fully understand for future if the 80% Zs Is breached of the OCPD I guess it would be bad design to rely on the RCD as fault protection ? But if you in a position where you adding to an existing install and it can’t be helped then using RCD for fault protection is Ok. I know it’s probably a bad design if starting from scratch. 

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Some may consider it bad design but BS7671 allows an rcd for fault protection.

NozSpark said:

You can't just add a RCD to get your allowed Zs increased (for fault protection) as you still have to ensure that your (R1+Rn) is low enough so that you have adequate overcurrent protection too

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Thank you for this. Is there a table that tells us the allowed vaules ( like with ZS) ? 

NozSpark said:

You can't just add a RCD to get your allowed Zs increased (for fault protection) as you still have to ensure that your (R1+Rn) is low enough so that you have adequate overcurrent protection too

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How does a "low enough" R1+Rn affect overcurrent protection. 

Fleeting said:

How does a "low enough" R1+Rn affect overcurrent protection. 

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More to the point,,,, how do you think that an excessively high circuit R1+Rn might affect the operation of a MCB with regard to over current?

NozSpark said:

More to the point,,,, how do you think that an excessively high circuit R1+Rn might affect the operation of a MCB with regard to over current?

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You answer with a question without explaining your statement? 

Fleeting said:

You answer with a question without explaining your statement? 

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MEOW .................

NozSpark said:

More to the point,,,, how do you think that an excessively high circuit R1+Rn might affect the operation of a MCB with regard to over current?

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your mixing overload current and fault current

I was discussing this the other day with somone, and we came up with a few interesting observations, now an RCD operates when it detects an imbalance in the circuit caused by either phase or neutral going down to earth, now ok we all agree this is a fault and so the RCD should trip.However suppose we get a dead short, phase to neutral, this too is a fault, but one which won't trip and RCD! I think there's far too much reliance on these devices these days, and even worse  in my opinion is the test scenario that if it trips at the outgoing terminals then the RCD is good, no matter wether or not it trips at the point of use.

Now I know that in reality no decent spark would leave a device in circuit that failed to trip at point of use, but my point is this, if you take it that according to the regs and testing procedures, an RCD behaving in the manner  I have described would still be fit for service, how I wonder would the coroner take it at an inquest?  Imagine it, ' well your honour I knew there was a chance someone could be killed, but the regs said it was fine so I left it and sure enough the man died mowing his lawn' . Would you want to be that spark, I don't think I would.

Fleeting said:

You answer with a question without explaining your statement? 

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Given the OP is a trainee is it not better to prompt the OP for an answer and let them think about what they are doing than hand it all out on a plate or should should we offer a multi choice answer and make it like an exam were the answers are taught from past papers. It's all to easy to be given the answer and learn nothing

UNG said:

Given the OP is a trainee is it not better to prompt the OP for an answer and let them think about what they are doing than hand it all out on a plate or should should we offer a multi choice answer and make it like an exam were the answers are taught from past papers. It's all to easy to be given the answer and learn nothing

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The OP didn't pose the statement and I am also interested in the answer.

NozSpark said:

More to the point,,,, how do you think that an excessively high circuit R1+Rn might affect the operation of a MCB with regard to over current?

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Your response

Fleeting said:

You answer with a question without explaining your statement? 

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My response

UNG said:

Given the OP is a trainee is it not better to prompt the OP for an answer and let them think about what they are doing than hand it all out on a plate or should should we offer a multi choice answer and make it like an exam were the answers are taught from past papers. It's all to easy to be given the answer and learn nothing

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Then your response

Fleeting said:

The OP didn't pose the statement and I am also interested in the answer.

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Just to make it all a bit clearer for you and now we await the OP's response as I suggested

UNG said:

Your response

My response

Then your response

Just to make it all a bit clearer for you and now we await the OP's response as I suggested

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I'm not asking for a response from the OP.

UNG I’m not sure if you are expecting an answer from me. Not sure if you see my other posts but I’m sure the regular chaps that push me in the right direction to an answer would back up the fact I’m not spoon fed answers. I give my thoughts on the questions, there’s no point in being given an answer as I won’t learn. I just like clarification and pointing in the right direction. 

Traineeboy said:

UNG I’m not sure if you are expecting an answer from me. Not sure if you see my other posts but I’m sure the regular chaps that push me in the right direction to an answer would back up the fact I’m not spoon fed answers. I give my thoughts on the questions, there’s no point in being given an answer as I won’t learn. I just like clarification and pointing in the right direction. 

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The comment I made to @Fleeting was regarding the question @NozSpark posted earlier in the thread which @Fleeting wanted him to answer without you having the opportunity first to give your thoughts

I''m well aware of your other posts and responses to questions but it would appear some posters on here are not familiar with not spoon feeding answers to trainees and were forcing an issue