Pump wired in 2.5mm on a shower circuit 10mm on Type B 40 acceptable ?

[Rob678]

Good evening all, I’m currently part way through my apprenticeship and we are working on IB, IN, IZ and IT and volt drop today. And it got me thinking I have seen a circumstance at work, where:
A shower has been wired in 10mm T+E on a 40A breaker. But then from the shower isolator switch a 2.5mm cable has been terminated and run to a pump (pump to remove water going down plug hole). In my mind from what I’m learning currently I would say this wouldn’t be okay due to the cable being undersized for the MCB (IB/IN/IZ) being used. I have heard it at work this arrangement being referred to as “downstream fusing” ?? Can’t find anything in the regs and I feel like it’s wrong.
However I’ve also been told that because the pump would never draw more amperes then the 2.5mm it is okay. I feel like this is incorrect and am looking for some more opinions.

thanks in advance guys and gals

 

[Fazz711]

Is there a fuse on the 2.5?

 

[brianmoooore]

However I’ve also been told that because the pump would never draw more amperes then the 2.5mm it is okay.

Won't when it's all in working order, but that applies to any correctly fused circuit.
If it stalls, it will draw a few times it's working current, and if it cooks completely it can go completely short circuit.

 

[Pretty Mouth]

Good evening all, I’m currently part way through my apprenticeship and we are working on IB, IN, IZ and IT and volt drop today. And it got me thinking I have seen a circumstance at work, where:
A shower has been wired in 10mm T+E on a 40A breaker. But then from the shower isolator switch a 2.5mm cable has been terminated and run to a pump (pump to remove water going down plug hole). In my mind from what I’m learning currently I would say this wouldn’t be okay due to the cable being undersized for the MCB (IB/IN/IZ) being used. I have heard it at work this arrangement being referred to as “downstream fusing” ?? Can’t find anything in the regs and I feel like it’s wrong.
However I’ve also been told that because the pump would never draw more amperes then the 2.5mm it is okay. I feel like this is incorrect and am looking for some more opinions.

thanks in advance guys and gals

Is the 2.5mm wired directly to the pump flex? or is it terminated in a socket or FCU, which the pump is plugged/wired into?

 

[Julie.]

There are two reasons for a fuse/mcb they are:

1) fault conditions - which must exist always, and be capable of operating for all points on the circuit within the time determined by the sort of circuit; and be located at the source. (ADS)
2) overload conditions - which is not required if it's a fixed load, and when required could be either at the source OR further down the circuit.

So taking the shower/pump circuit

2) The pump is not a fixed load - so needs overload protection - to protect the cable (2.5mm^2) - so this protection (such as a 3A , 13A fuse) can either be at the start of the 2.5mm^2 run or further down.

1) This whole circuit upstream of the "pump fuse" needs fault protection at the source, and it must operate within time (i.e. low enough Zs) so less than around 0.88 ohm for a 40A MCB at ALL points of the circuit upstream of the "pump fuse".

If you measure the Zs at the pump fuse and it's less than 0.88 ohm, then it's ok, the 2.5mm^2 cable (and the 10mm^2 cable) is protected against faults, by the 40A MCB, and overload (on the 2.5mm^2 cable) by the downstream (3A.....13A) fuse.

If you measure the Zs at the pump fuse and it's greater then the 0.88 ohm, then there is insufficient protection for faults, and the fuse needs to be relocated further upstream.


Edit.

It's actually clearly stated in the regs, Section 411.3.2 covers ADS giving the design Zs and timescales (TN...TT systems and various voltages etc) whilst overload is in section 443 - the omissions are 433.3.1 and position in 433.2 (check 433.2.2)