Max Zs for type 'C' BS61009

Merl · Nov 28, 2011

Can anyone put my mind straight on the Max Zs value for a 32A type 'C' BS61009. The BRB states the max measured value for this type of breaker as 0.72 ohms. Using the rule of thumb ( x 0.8 ) this brings the value to 0.57 ohms. Is this 0.57 ohm maximum measured value to meet the 0.4 & 5 second disconnection times in a short circuit fault conditon? Am I right in thinking that any measured value above this 0.57 ohms the breaker wouldn't trip within the required times??

The reason I'm asking is, I've just done some testing and the sockets are protected by 32A type 'C' RCBO's and a number of Zs test readings are above the 0.57 ohm maximum permitted value, but would be below the max permitted value of a type 'B' RCBO and well below the 1667 ohms for earth fault protection.

Any advice would be appreciated!

Cheers!

spark 68 · Nov 28, 2011

C= X10 In

(10*32) = 320

230/320 = 0.72

0.72*0.8 = 0.58 ohms

But your RCD is taking care of the Zs, if it is a 30mA type, the max Zs = 1667 ohms

Merl · Nov 28, 2011

Cheers spark68.

I had a good think about what I had asked after I posted the question. Yes, I looked at the time/current characteristic graph in the BRB and came to the conclusion that it's going to be a potentially expensive fix for the customer!

I can't understand why these type C RCBO's were were fitted when the place was refurbished and rewired about 8 years ago! I checked back to a 16th ed OSG and the values haven't changed. They mustn't of complied then, or were borderline!! Apart from this problem it's a first class job that's been done!

spark 68 · Nov 29, 2011

Hi Merl,

I don't see what you are getting at ?

As it is an RCBO your disconnection times (40mS) will be met, unless your Zs is above 1667 ohms.

The max Zs for the breaker (MCB type C) is for the 0.4 s disconnecton time (in reality much quicker @ 320A), this will be taken care of by the RCD part of the RCBO, so what's the problem ?

Merl · Nov 29, 2011

Hi spark68

Yes the 40mS disconnection time will be met by the RCD part of the RCBO for an earth fault current, but what about a short circuit fault current L-N?

I dont think I explained it very well.

I'm a fellow ELECSA member too.

spark 68 · Nov 29, 2011

This should not matter as your breaker will still trip if a short circuit/overcurrent occurs, it is still a 32A breaker, the C type doesn't affect the In of the OCPD.

Breakers are made up of two parts the magnetic and the thermal part, the C refers to the magnetic part (for surges etc), the Thermal part will still trip in the event of a fault. How quick depends on the magnitude and duration of the overload/fault current.

Merl · Nov 29, 2011

The light has just been switched on in my brain!!!

Thanks spark68!

Yes I've had totally the wrong idea for some reason!!

Zs = Ze + R1 (L) + R2 (E). The earth fault path! Geeeez, basic stuff really!!

I feel a bit of a numpty now like.

Thanks again spark68!

spark 68 · Nov 29, 2011

It's That time of night mate.

jamie-spark · Dec 1, 2011

Merl said:
Hi spark68

Yes the 40mS disconnection time will be met by the RCD part of the RCBO for an earth fault current, but what about a short circuit fault current L-N?

I dont think I explained it very well.

I'm a fellow ELECSA member too.

Merl. I think you are correct in your line of thinking. If you look at page 44 in the OSG. I think you are limited by the length of the circuit as impedance between LN. AS for a type c It is not permited due to line to neutral impedance

jamie-spark · Dec 9, 2011

Merly whilst looking through th OSG today i found this on page 55 .

7.2.4 RCDS
Where circuits have residual current protection, the limiting factor is often the maximum loop impedance that will result in the operatiob of the overcurrent device within 5 secondsfor a short circuit (LINE TO NEUTRAL)lfault.

If i remember correct that is0.7