UK Relevance of Table 41.3 where Additional Protection is present?

[GroovyG]

Reading of 411.3.2 and following suggests that compliance for disconnection <400ms may be obtained using either MCB or RCD. Where an RCD (RCBO in this case) is used for Additional Protection, is compliance with Zs specified in Table 41.3 still necessary?
The situation: new installation by A.N.Other in a timber shower/toilet block on a camping facility. Initially 8 no. hand dryers on a B20 wired in 2.5 T&E radial, 1.25mm flex drops from local FCU to each dryer. Repeated tripping found even with only a single dryer activated led to installer converting radial to ring, protected by C32/r30mA RCBO, and moving 2 of the dryers to a different circuit. The tripping persisted. When I was asked to look at it, I was unable to identify cause of tripping, as nothing showed up in any tests. I concluded it to be an initial burn-in problem, as no more tripping was reported after a couple of months in service.
The problem: while rectifying other poor work, I found the highest R1+R2 reading at the most remote dryer to be 0.51Ω, which with Ze of 0.42Ω puts the Zs well above the Table 41.3 limit for a C32 device.
The question: does this matter?
Thanks for your thoughts.

 

[westward10]

No the RCD side will also offer fault protection.

 

[GroovyG]

Should we not take into account the possibility/probability of the RCD mechanism going 'sticky', as we all know they can, in which case the 400ms disconnection time would not be achieved?

 

[westward10]

You don't need to take that into account.

 

[DefyG]

possibility/probability of the RCD mechanism going 'sticky'

When its tested every 6months (ha ha!) such faults may be reported and the RCBO replaced - wont it?

 

[newfutile]

I would be asking why it's on a C32,while it is permitted I don't think it's a great design to exceed maximum ZS and rely only on the (RCD) Residual Current Device partof the rcbo

 

[cireland]

You could try putting it on a C20. I would have to agree with not relying on RCD to trip.

 

[newfutile]

I was more thinking why a C curve breaker , a 32 is most likely needed in case all get put on together, but the motor is probably not much of the load ,so i would guess not too much inrush current to require a C device, you could measure with a clamp meter with a max functon to test this

 

[MFS Electrical]

Reading of 411.3.2 and following suggests that compliance for disconnection <400ms may be obtained using either MCB or RCD. Where an RCD (RCBO in this case) is used for Additional Protection, is compliance with Zs specified in Table 41.3 still necessary?
The situation: new installation by A.N.Other in a timber shower/toilet block on a camping facility. Initially 8 no. hand dryers on a B20 wired in 2.5 T&E radial, 1.25mm flex drops from local FCU to each dryer. Repeated tripping found even with only a single dryer activated led to installer converting radial to ring, protected by C32/r30mA RCBO, and moving 2 of the dryers to a different circuit. The tripping persisted. When I was asked to look at it, I was unable to identify cause of tripping, as nothing showed up in any tests. I concluded it to be an initial burn-in problem, as no more tripping was reported after a couple of months in service.
The problem: while rectifying other poor work, I found the highest R1+R2 reading at the most remote dryer to be 0.51Ω, which with Ze of 0.42Ω puts the Zs well above the Table 41.3 limit for a C32 device.
The question: does this matter?
Thanks for your thoughts.

If I encountered this on an EICR I’d still code it as a C3. It is technically compliant, however the Zs exceeding the MCB type and rating is just bad design.
Having said that I’ve rectified many a C2 for high Zs by changing to an RCBO.

Would the loads allow for a reduction to a C20?

 

[westward10]

Bad design or what you are dealt with. I need a type D 32A circuit but the Zs the DB I am connecting to is 0.40. My design is sound but I have to deal with what I have you could not Code it, ADS is satisfied with an RCBO.

 

[DefyG]

32A mcb protecting a 2.5 t&e cable ????

 

[brianmoooore]

32A mcb protecting a 2.5 t&e cable ????

Every RFC with a spur.

 

[DefyG]

32A mcb protecting a 2.5 t&e cable ????

Duh!!! Just re-read Op's original post, it was converted to a

led to installer converting radial to ring,

RFC!