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Discuss Earth Fault Loop Impedance... in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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Hi there, I’m just hoping someone could clear something up for me at all. I’m just working my way through some test questions and I’ve come across one in which isn’t making sense to me.

Q. Calculate the maximum earth fault loop impedance permissible for a 30mA RCD used to protect a circuit connected to a TN-S supply.

A. 230/0.03 = 7666ohms (if a TT system, use 50volts, but must use 230volts for a TNS and TNCS system)

Does this all seem correct, or should the question be asking for PFC rather than impedance? If I am misunderstanding, then a short explanation would really put me on the right track with this.

Thanks again in advance for your invaluable help forum.
 
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Hi there, I’m just hoping someone could clear something up for me at all. I’m just working my way through some test questions and I’ve come across one in which isn’t making sense to me.

Q. Calculate the maximum earth fault loop impedance permissible for a 30mA RCD used to protect a circuit connected to a TN-S supply.

A. 230/0.03 = 7666ohms (if a TT system, use 50volts, but must use 230volts for a TNS and TNCS system)

Does this all seem correct, or should the question be asking for PFC rather than impedance? If I am misunderstanding, then a short explanation would really put me on the right track with this.

Thanks again in advance for your invaluable help forum.
R=E/I wher R=1667,E=50V, I=0.03. hope this helps.
 

telectrix

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I still don't see the need for them changing it.
without changes how would the IET top up their pension funds?
 

davesparks

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I still don't see the need for them changing it.
I can't see any need for it either, if a circuit has got to 1667ohms there's something seriously wrong, let alone 7667ohms!

Plus people seem to blissfully ignore manufacturers requirements. For example if you look at the instructions for a schneider RCBO it stipulates 100ohms maximum.
 
B

Bobster

I can't see any need for it either, if a circuit has got to 1667ohms there's something seriously wrong, let alone 7667ohms!

Plus people seem to blissfully ignore manufacturers requirements. For example if you look at the instructions for a schneider RCBO it stipulates 100ohms maximum.
Been a while since I last used a Schneider RCBO but they also stated that they couldn't be used on TT installations.

I think with the new generation this caveat has been removed.
 

davesparks

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Been a while since I last used a Schneider RCBO but they also stated that they couldn't be used on TT installations.

I think with the new generation this caveat has been removed.
I think it's still the same, ive just fitted a board with the new plug in neutral type, I'll see if any of the instructions are still hanging around tomorrow.
 

Strima

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Been a while since I last used a Schneider RCBO but they also stated that they couldn't be used on TT installations.

I think with the new generation this caveat has been removed.
Schneider make both single and double pole RCBOs so you need to be careful which ones you order.
 
B

Bobster

Schneider make both single and double pole RCBOs so you need to be careful which ones you order.
Aware of this, however the original ACTI9 RCBOs wouldn't operate without a decent reference to earth through the functional fly lead. Meaning in poor TT conditions, they may never operate.
 
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