Suitability of circuit accessories.

[Leezo]

Hi all,

Hopefully this is being posted in the right place, apologies if not or if my question has been previously answered.

I'm currently having a bit of a disagreement with my supervisor having carried out an inspection which is for all intents & purposes, a condition report on a living quarters within a military base.

I have reported a 32A single phase circuit feeding 2 air conditioning split units where each split unit has local isolation in the form of a 20A rotary isolator. I didn't think twice about listing this as a defect, I was taught from day 1 that accessories must have a current rating equal to or greater than the design current of the circuit for obvious reasons. My supervisor has subsequently rubbished this claiming that due to there being fixed equipment connected to the load side of the isolator that does not run close to 20A that the problem is mitigated. Whilst I see his point I still don't think its right & have been looking in the regs for a definitive answer to the question, so far I've only found 512.1.2 under selection & erection...

512.1.2 Current
Every item of equipment shall be suitable for: (i) the design current, taking into account any capacitive and inductive effects, and (ii) the current likely to flow in abnormal conditions for such periods of time as are determined by the characteristics of the protective devices concerned.

Just wondering if anyone has any thoughts on this or can point me in the direction of a reg that categorically proves me or him right.

Thanks in advance!

Nick.

 

[Strima]

For a fixed load the design current would be that of the load not the protective device rating.

As there's no risk of overload I would see no issue and not code it.

 

[telectrix]

agree ^^^^^^. even if the circuit is running at full capacity of 32A, the isilator can't pass a current greater than it's load demand (except in the case of a fault condition in the load or it's connection cable ).

 

[Leezo]

agree ^^^^^^. even if the circuit is running at full capacity of 32A, the isilator can't pass a current greater than it's load demand (except in the case of a fault condition in the load or it's connection cable ).

But if there is a fault condition then the load could draw in excess of 20A, ie the rating of the isolator for a sustained amount of time while not pulling enough current to operate the 32A protective device thus potentially causing damage to the under rated isolator & therefore a fire hazard, seems like common sense to me?

 

[Strima]

Accessories are designed to take into account short duration fault conditions with no adverse effects.

 

[Leezo]

For a fixed load the design current would be that of the load not the protective device rating.

As there's no risk of overload I would see no issue and not code it.

But if there is a fault condition then the load could draw in excess of 20A, ie the rating of the isolator for a sustained amount of time while not pulling enough current to operate the 32A protective device thus potentially causing damage to the under rated isolator & therefore a fire hazard, seems like common sense to me?

 

[snowhead]

The 20a isolator will already be rated for overcurrent.

If there was sufficient current ot set fire to the Isolator the Air Con unit would probably already be on fire.

 

[Leezo]

The 20a isolator will already be rated for overcurrent.

If there was sufficient current ot set fire to the Isolator the Air Con unit would probably already be on fire.

Yes, rated for some overcurrent at 20A.

Have you never seen a circuit accessory that's overheated &/or melted over a period of time? I know I have.

 

[Julie.]

But if there is a fault condition then the load could draw in excess of 20A, ie the rating of the isolator for a sustained amount of time while not pulling enough current to operate the 32A protective device thus potentially causing damage to the under rated isolator & therefore a fire hazard, seems like common sense to me?

How could this "fault" condition occur?

- if it's a short circuit fault, then you design the protection, cable etc to give a Zs low enough to ensure the protective device trips quick enough - if this isn't the case, it's not an isolator issue it's a proactive device issue.

- If it's an overload condition (not a fault) , then the fixed load (such as a 4 ring hob, has to switch a 5th or 6th ring on in order for it to occur!! - so isn't possible), in the case of motors etc, there is a possibility of overload, but usually there is motor protection locally, such as the case with an air-con unit.

 

[Leezo]

How could this "fault" condition occur?

- if it's a short circuit fault, then you design the protection, cable etc to give a Zs low enough to ensure the protective device trips quick enough - if this isn't the case, it's not an isolator issue it's a proactive device issue.

- If it's an overload condition (not a fault) , then the fixed load (such as a 4 ring hob, has to switch a 5th or 6th ring on in order for it to occur!! - so isn't possible), in the case of motors etc, there is a possibility of overload, but usually there is motor protection locally, such as the case with an air-con unit.

I didn't design the cct Julie so none of that is down to me or part of the issue. Any number of faults could occur in an air con unit, its not just a motor.