high ZS what would you do?

[Superspark23]

Was having this discussion today with the guy i was working with.

You went and did an PIR and found several of the ZS current over the max ZS alloud by the regs so wont disconnect in 0.4s. I was told on the 2392 tesing course that it would be okay to put these circuits on an rcd, as this will allow the circuit to disconnect within 0.4s. what do you think of this practise. Personnally i dont agree with it nor did the guy i was working with.

 

[Sickmick]

It's a lazy fix, better to find out why they're over max Zs. What's the Ze?

 

[Guest123]

This type of question comes up on 2392/91 question papers from time to time where it tells you the measured Zs for a particular circuit exceeds the maximum permitted in BS 7671 and to state 3 ways of combating this

1. Downsize the protective device
2. Increase the size of CPC
3. Install additional RCD protection.

So, installing additional RCD protection whilst it may be seen as a 'quick fix' is an acceptable solution, for remedial works after a PIR anyway. If it were a new install I would be thoroughly inspecting & testing the circuit as IMO there is no reason for the Zs values not to be met, unless it's a TT install of course.

 

[SirKit Breaker]

Was having this discussion today with the guy i was working with.

You went and did an PIR and found several of the ZS current over the max ZS alloud by the regs so wont disconnect in 0.4s. I was told on the 2392 tesing course that it would be okay to put these circuits on an rcd, as this will allow the circuit to disconnect within 0.4s. what do you think of this practise. Personnally i dont agree with it nor did the guy i was working with.

Classic BS7671 again. Tells you one thing, then tells you something else. I would also want to bottom the original problem. Just fitting a RCD cures the symptoms but not the disease so to speak.

Cheers..............Howard

 

[Deleted member 9648]

It would depend on the reasons why the Zs exceeds the max for the overcurrent device. It may be that a type C has been fitted to a circuit to allow for equipment with a high inrush current,when the measured Zs would be OK with a type B. This is simply because of a change of overcurrent device and not a design or other problem. IMO in such circumstances it would be acceptable to rely on an RCD to meet disconnection times.

 

[Superspark23]

I agree with this but over time the max zs values have been lowered with amendments to the regs. So a once acceptable ZS reading may no longer be, so providing you have evidence of this through past test results then i would only put the circuit on an rcd. Another thing some where in the regs it states that an RCD shouldn't be the sole protection of the circuit.

 

[Guest123]

Another thing some where in the regs it states that an RCD shouldn't be the sole protection of the circuit.

Thats right, you'd still need the overcurrent element of the fuse/MCB.

 

[Deleted member 9648]

I agree with this but over time the max zs values have been lowered with amendments to the regs. So a once acceptable ZS reading may no longer be, so providing you have evidence of this through past test results then i would only put the circuit on an rcd. Another thing some where in the regs it states that an RCD shouldn't be the sole protection of the circuit.

That is referring to overcurrent protection,there is no reason why an RCD cannot be the sole means of earth fault protection.

edit-(Lenny types quicker than me!)

 

[Johnny-G]

I've never been keen on just sticking it on an RCD so I'd find out what's causing it, after all, we're supposed to be professionals who get paid to rectify issues like these.

 

[Superspark23]

Exactly and what if the rcd fails? then you have a dangorous scenareo