RCD Calculations

[mmax6870]

Hi All,

I would be very gratefull if anyone can clarify the correct method to calculate correct rating of RCDs, as I have read numerous threads which seem to differ.

I have just completed my Level 2 C&G 2330 and understand the cable calcs, but haven't asked my tutor this question yet. Here's my thinking (right or wrong) but I would welcome any formulae that is out there.

If using a 63A RCD to protect numerous circuits for earth fault current, then the total MCB Amp ratings (for these cicuits) should not exceed 63A, as diversity should have already been carried out to determine MCB ratings?

Thanks in advance for your help

 

[rocker]

Yep apply diversity on the circuits protected by the RCD to the rating of the RCD as you would on the main incomer.

I personally don't see the point in using anything less than 80 or 100A RCDs - the protection is coming from the main incomer so even if you have a 63A main switch, the RCD is there for the 30mA protection NOT overload protection so using a bigger one can't hurt, and they all cost the same. I'm probably wrong though I bet theres a reason for it.

 

[mmax6870]

Thanks Rocker

So, if someone has a 63A RCD which protects 2 x 32A ring final circuits say, adding a 6A lighting circuit would not be permissible and an 80A RCD would be required?

I assume that is also the case then for split load boards?

Thanks

 

[m4tty]

Most split load boards ive seen come loaded with breakers which when added together far exceed the rating of the rcds. So I suppose diversity is applied to these. Cheers

 

[rocker]

No, as you haven't applied diversity to the circuits. Have a look at appendix 6 of your on site guide, you would be able to get more on than that. I'd still use an 80A if it were me. Just out of interest, what is the rating of your main switch?

 

[acat]

Correct diversity must be applied in most split boards these days if you just add up the value of all the MCBs you will find it exceeds the 100A main fuse rating


Chris

 

[mmax6870]

No, as you haven't applied diversity to the circuits. Have a look at appendix 6 of your on site guide, you would be able to get more on than that. I'd still use an 80A if it were me. Just out of interest, what is the rating of your main switch?

It is purely a hypothetical question, so no main switch rating I am afraid. What effect would that have?

 

[rocker]

None for the purposes of this question, but in the broader sense you woud need to take both of your figures after diversity for both RCDs (if a 17th board) and make sure they are both OK for the main switch, and your RCD is not going to provide overload protection so you would need to make sure your main switch is sufficient.

 

[mmax6870]

None for the purposes of this question, but in the broader sense you woud need to take both of your figures after diversity for both RCDs (if a 17th board) and make sure they are both OK for the main switch, and your RCD is not going to provide overload protection so you would need to make sure your main switch is sufficient.

So, with dual RCDs fitted the maximum total RCD ratings should not exceed 100A, if that is the main switch rating?

Cheers

 

[rocker]

Ha. Hmmm. No, not really, despite what I just said, what you want to do is apply diversity AGAIN to all the MCBs together, ignoring the RCDs, when selecting the main switch.

 

[mmax6870]

Ha. Hmmm. No, not really, despite what I just said, what you want to do is apply diversity AGAIN to all the MCBs together, ignoring the RCDs, when selecting the main switch.

I had a feeling it wasn't going to be straightforward! Is there a specific value applied for this or a section in the OSG which gives details, as I haven't come across this yet?

Cheers

 

[ezzzekiel]

a rough rule of thumb is to take full load of highest rated circuit then total up all remaining breakers in use and take 40% value as applied diversity

 

[JUD]

I had a feeling it wasn't going to be straightforward! Is there a specific value applied for this or a section in the OSG which gives details, as I haven't come across this yet?

Cheers

a rough rule of thumb is to take full load of highest rated circuit then total up all remaining breakers in use and take 40% value as applied diversity

As ezzzekiel said.

Take a look at Table 1B on page 97 of the OSG, specifically number 9.

Standard arrangement of final circuits in accordance with Appendix 8....100% of current demand of largest circuit + 40% of current demand of every other circuit.

 

[mmax6870]

Thank you everyone for your help and input.

I will keep my eyes open when out and about to see how many boards meet these figures!


Cheers

 

[mmax6870]

Back again!

Thought I would check my DB (which has been in about 5 years) and see if calcs work out. Please advise if I have worked out correctly.

Main switch 100A this feeds

• 8 MCBs 32A x 1, 16A x 2, 20A x 1, 6A x 4

• RCD 80A then covers 5 more MCBs 32A x 3, 20A x 1, 16A x 1

Working on 100% of highest rating circuit (32A) then 40% of remaining circuits (208A @ 40% = 83.2A) giving a total of 115.2A which is higher than main switch. Therefore my DB is out of existing regs?

Thanks for your patience!