RCD Earth Loop Impedance

[magicman]

Help!
Hi Guys,
can anyone shed any light on a query regards the max. ELZ for a 30mA RCD protected circuit?

Reg. 413-02-16 states that the max. ELZ for a 30mA RCD should be 50V/0.03 mA = 1666 ohms.

I have a ring main protected by a 32A/32mA RCBO (ie an MCB/RCD combination)
Table 41B2 lists the max. ELZ as 1.5ohms (for a type B).

Why is the Regs. differentiating between an RCBO (1.5ohms) and an RCD (1666ohm) and say a 32A type B MCB combination.
(Bearing in mind you may not be able to achieve a ELZ of 1.5ohms on a TT system, when you may want to use an RCD/MCB combination)
Or have I got my K in a twist!!
Thanks Guys
magicman

 

[ban-all-sheds]

The reason is that in TN systems it is preferable for RCBOs to operate in overcurrent mode when providing indirect shock protection. When they are working in overcurrent mode they are voltage independent, whereas in RCD mode they are not; they need a large enough voltage being applied to drive the circuitry inside. The IEC stipulate that voltage dependent RCDs should operate at voltages as low as 50V, so that the effects of a collapsing voltage in a fault condition are nullified. But if you get an open-circuit neutral, it is possible that an RCBO will not operate in RCD mode with a fault to earth, so the design for a TN system is supposed to ensure that RCBOs operate in overcurrent mode for indirect shock protection, and for this reason the earth fault loop impedence restrictions are the same.

 

[plugwash]

i thought broken neutrals were the whole reason rcbos have an earth connection

 

[Lectrician]

And many RCD's and RCBO's will trip as soon as the supply neutral is removed.

The 50v taken from the regs is to ensure a voltage on exposed conductive parts doesn't exceed the safe touch voltage under a fault condition.

 

[magicman]

Thanks Guys!
I only signed up to DIYnot.com on Fri. and had a reply posted on Sat.-impressive!!
Thanks "ban-all -sheds" for your excellent reply.

Since posting my question, I have since found an article in -Earthing- "Snags and Solutions" (by the NICEIC)-
as you correctly pointed out, a TT system is treated differently to a TN one and the NICEIC considers that a reading up to 100 ohms is considererd acceptable, above that, it states it could be unstable due to ground resistance changes etc.
It is interesting to note that the addition of RCBO's (TN systems only)to table 41B2 has only had added recently, as it does not appear in an earlier addition of the REGS.
Thanks again!
magicman