Fusing downstream with very short cable length

[Ed Knights]

Hi, first post newbie here.

I have a question relating to what I am told is a grey area in both 7671 and low voltage assemblies regs. I have a customer with some LV industrial control panels, 100A fused isolator with 25mm main singles from isolator down to motor drive, but off the isolator terminals are also some short 6mm cables that go immediately to 1A fuses (within 3 or 4 inches). I am aware obviously this contradicts the requirement for conductor CSA rating to be > upstream protective device rating, but also on the other hand have been told in the past this is fine to do as long as the under-rated cable is of sufficiently short length, there is no danger of mechanical damage to the cable, and the design load of its downstream circuit is suitably low - I believe all of those to be true in this case. So the opposite of de-rating I guess! Has anyone had a similar quandary in the past, and if so were able to find a definitive answer within the UK regs?

Entire circuit is enclosed within a steel control panel. Drawing attached, as well as layout sketch of the installation, unfortunately I don't have photos of the actual installation sorry.

Thanks in advance
Ed

 

[timhoward]

See 433.2.2 and 433.3.1 especially ii)

Main alarm bell - is the neutral fused?
The circuit diagram says not, the text and other picture says so.

 

[freddo]

This sort of thing is done all the time for voltage inputs of sub meters etc. The main problem I've seen is undersized breaking capacity of the fuses.

 

[Bluetit]

Most panels at my current workplace run some alarmingly small cable off the incomer to door indicators, fuses add a point for possible confusion around the status of the panel it's argued. Admittedly these are designed to 60204-1 rather than 7671, but they're largely similar on matters such as this.
It has, and seemingly always will be a hot topic. The means of termination is also worthy of note, a very large and very small cable wedged in one terminal is not ideal.

 

[Ed Knights]

See 433.2.2 and 433.3.1 especially ii)

Main alarm bell - is the neutral fused?
The circuit diagram says not, the text and other picture says so.

Thanks Tim, yes the neutral is fused now, there was a fuse carrier with a solid link when we first looked at the panels but have advised the customer to drop fuses in instead - hence why the drawing shows no fuse on the N side.

 

[mainline]

Why would the neutral be fused ? In the case of an overcurrent, the neutral fuse could go, leaving the L still energised.

 

[westward10]

Thanks Tim, yes the neutral is fused now, there was a fuse carrier with a solid link when we first looked at the panels but have advised the customer to drop fuses in instead - hence why the drawing shows no fuse on the N side.

That was bad advise they make neutral links for a reason.

 

[Ed Knights]

That was bad advise they make neutral links for a reason.

My mistake, I'm being told this was another circuit that took 400v off 2 phases to feed a 400-110v transformer, this is where a solid link was found in one of the fuse carriers and advised customer to replace with fuse as is a live phase, the neutral link was left in on the single phase circuit pictured down to the heater. Corrected the below

 

[westward10]

Fair enough.

 

[timhoward]

Thanks Tim, yes the neutral is fused now

I was trying to say it shouldn’t be! Sorry I wasn't clearer.

 

[Ed Knights]

I was trying to say it shouldn’t be! Sorry I wasn't clearer.

No worries, my fault as I am not-very-qualified and passing on 3rd hand information!!! I appreciate your help and totally understand why the neutral shouldn't be fused now.

 

[freddo]

Here is some 1.5mm2 connected to 1600A busbars :

 

[Ed Knights]

Here is some 1.5mm2 connected to 1600A busbars :
View attachment 117717

Excellent, the 1.5 cable is its own fuse

 

[freddo]

Excellent, the 1.5 cable is its own fuse

I can imagine it just disappearing (loudly) under short circuit conditions, then the resulting blast/plasma etc. casing the busbars to flash over damaging everything before taking out the ACB. The conductor on the other side of the bolt is a 240mm2 Cu-Flex.