Zs too high but RCD protected. Blond moment

[17thman]

Ok this may just clear up an area for me

What if your Zs is too high for automatic disconnection within required time, say 0.4 secs but the circuit is rcd protected and the rcd test reveals disconnection time of say .17ms for the 5x test. Then what does it realy matter about the high Zs?

411.3.2.2 staes that a protective device shall automatically interupt the supply of the line conductor blah blah blah. but an rcd is a protective device.

Any view on this would be appreciated.

 

[Spark123]

Have a read of 411.4.9

 

[17thman]

thx spark.

I have read it and the figures in 41.5 are easily obtained in a tn system. Zs does not influence overcurrent situations of short circuit and overload nature. so i am still left with the fact that if Z|s it a little too high for auto disconnection then The RCD would sort it anyway. An RCD is a protective device as detailed earlier. All I can think of is the fact that in the situation in the original post then the rcd is additional protection and the primary protection device must comply but where is that detailed? It may seem a little pedantic but it just helps me understand fully.

 

[Spark123]

It used to be the preferred in the 16th that the Zs should be low enough for the overcurrent protective device to operate, think it is now just table 41.5 via reg 411.4.9 in the 17th ed.

 

[17thman]

I think what i am trying to get at is that the Zs is worthless if you have an rcd protected circuit and R1+ Rn is low enough for short circuit conditions to comply. In paractice i cannot see why i am wrong but I must be missing somthing

 

[Spark123]

Zs isn't really worthless, I'd expect any electrician who is working on a TN system to investigate any reading of significance regardless of the max Zs figure quoted in the table.
I think you are also getting at short circuit conditions? The 16th didn't used to have an upward time limit as such for a short between live conductors though if going above 5secs the cable manufacturer needed to be consulted. Not sure if that is still the case with the 17th.

 

[17thman]

yes, t= (K.sq x S.sq)/ I.sq takes care of short circuit conditions and yes i always would investigate Zs and also any value above tabulated figure. Just seems to be redundant somewhat if you have an rcd.

 

[Spark123]

An RCD doesn't protect against a fault between live conductors.

 

[17thman]

i know, that is why i'm saying Zs appears rudundant under earth fault conditions if you have an rcd. short circuit is another matter, proetction against thermal effects and damage to conductors

 

[NotHimAgain]

First, as others have said, if Zs is too high to meet the required disconnection time on a TN system the circuit should be investigated: checked for faults; poor design; etc.

Next you might like to read Paul Cook's Commentary on the IEE Wiring Regulations 16th Edition BS 7671 : 2001 (cheeky title given that BS 7671 is a joint BSI / IET (IEE) effort!).

Page 75 [4.5.7] spills the beans .

Basically, if you read between the lines, you will see that our standards makers have (or at least had - I not so sure now) a healthy distrust of RCBOs to BS 61009. The problem is that under fault conditions the supply voltage can collapse and if the RCBOs are electronic types they need voltage to work. The IEC have, to some degree, addressed this by requiring that these devices be able to work on driving voltages as low as 50 volts - but those JPEL/64 people are worried about failures in the neutral.

The up shot of all of this is that - on TN systems - they prefer that you design your circuits so that they will operate correctly using the overcurrent bit of the RCBO i.e. treat it as if it were just an mcb.

 

[17thman]

thanks nothimagain, I understand fully what you have said and have read up on it. it seems however that we get the use of words like "prefer" and the reason for the delayed response is that I have been working away for a week and keep getting 2 schools of thought. 1) is that you should do exactly what you say and design the circuit so that the ocpd will take care of the earth faault in the required time. the other is that if you have an RCD then what does is matter. I strongly prefer yours and my approach but consider that when we read things like "prefer" then there seems no real way of knowing whether the second approach is against regs.

I know it may sound pedantic but its interesting to me.