RCD mA calc

[martysparky]

Could someone please confirm how to calc the size of an RCD PLEASE?

I was asked this and am not sure, looked in the regs T 3A and think its V over RA but am not sure of this.

I'm only domestic but I need to get this straight in my nut.

Cheers guys.

 

[Guest123]

I'm a bit confused mate.....are you after calculating max Zs for an RCD using the safe touch voltage threshold i.e, 50V/0.03mA = 1667 ohms???

 

[martysparky]

I know the answer is 30mA but need to know how to arrive at that, sorry if I'm being a bit thick.

Hear is the question:
Earthing arrangement supported an RA 0f 1200 ohm's Based on a touch voltage of 50V. What size RCD is required for earth fault protection?

 

[JUD]

V = IR

A. 30mA...1200 x 0.03 = 36V √
B. 100mA...1200 x 0.1 = 120V X
C. 500mA...1200 x 0.5 = 600V X

Answer = A

 

[martysparky]

Don't know where I'm going wrong but I make it like this:

I = 50V over R 1200 = 0.04
V = 1200 x 0.04 = 48V = 30mA

 

[JUD]

Yeah, that works too.

I = V ÷ R

50V ÷ 1200Ω = 0.04A (40mA) so this tells you that you need an RCD rated less than 40mA i.e. 30mA.

 

[martysparky]

Yay, thank the help!

Couldn't find nowt about touch voltage and still can't mind.

 

[JUD]

Pages 50 - 52 BRB

 

[Guest123]

Hear is the question:
Earthing arrangement supported an RA 0f 1200 ohm's Based on a touch voltage of 50V. What size RCD is required for earth fault protection?

Makes sense now.

 

[martysparky]

sorry Lenny just put what I remembered at first but dug it out when I realized I was not being clear.