Earth Fault Loop Impedance...

[junksamiad]

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.

 

[JAWS]

Is it not 50/0.03 = 1667

 

[Wilko]

Hi - check out Reg 411.5.3 and Table 41.5

 

[UKPN]

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.

 

[davesparks]

Is it not 50/0.03 = 1667

For a TT system yes, and in the 17th edition this was also true for TN systems, but under the 18th edition for a TN system it is 230/0.03.

 

[davesparks]

R=E/I wher R=1667,E=50V, I=0.03. hope this helps.

Not for a TN system under the 18th edition

 

[Bobster]

For a TT system yes, and in the 17th edition this was also true for TN systems, but under the 18th edition for a TN system it is 230/0.03.

I still don't see the need for them changing it.

 

[telectrix]

I still don't see the need for them changing it.

without changes how would the IET top up their pension funds?

 

[davesparks]

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.

 

[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]

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]

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.

 

[Ian1981]

Not for a TN system under the 18th edition

Haven’t seen this change I’ll have to have a look

 

[Ian1981]

Schneider make both single and double pole RCBOs so you need to be careful which ones you order.

Why?
You don’t need DP rcbo’s on a TT
A DP main switch or 4 pole will give you your isolation

 

[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.

 

[davesparks]

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.

I wonder how may other RCBO's which have the earth reference connection have the same limitation?

 

[Wilko]

Not for a TN system under the 18th edition

Hi - just my read of it, but I had interpreted Reg 411.4.204 from the TN section to indicate we should use Table 41.5 from the TT section, giving 1667 Ohms again for 30mA. (?)

 

[Ian1981]

Hi - just my read of it, but I had interpreted Reg 411.4.204 from the TN section to indicate we should use Table 41.5 from the TT section, giving 1667 Ohms again for 30mA. (?)

Agreed I’ve done the same

 

[Strima]

Why?
You don’t need DP rcbo’s on a TT
A DP main switch or 4 pole will give you your isolation

If you read their instructions they state that their single pole RCBOs are not suitable for TT systems.

 

[Ian1981]

If you read their instructions they state that their single pole RCBOs are not suitable for TT systems.

Wonder why that is ?.
There is no requirement to switch/disconnect the neutral during over current conditions for a TT or TN arrangement except for certain requirements in part 7