Sub distribution supply fault protection question

[Paultrim123321]

Hi, carried out an inspection this week on an installation which consisted of two phase three wire to a small building from an electrical switch room. The supply is wired using 25mm LSF/SWA protected by BS-88 at the sub. At the building the supply is terminated into a 947-3 isolator and then reduced using various tails to smaller DB's and fused isolators. The tails vary between 4mm and 16mm depending on the circuit demand for example the single pole fused isolators are rated at 32A and fed by 4mm tails, the larger 9-Way CU's are fed using 16mm tails. Reading 433.3.1 would you consider this to be satisfactory as the fault protection at the load side of the install is satisfactory for the supply tails? IMHO I am thinking they need to provide local protection at the installation and then size the tails accordingly? This install is approx. 25 years old and no history of overload. I clamped the load whilst carrying out the inspection and the install was only drawing 6A. P.S The two phase supply is another question for later

 

[Dizzy_Maskell]

If you are asking this question are you sure your are competent to carry out inspections. ????????

 

[Engineer54]

If you are asking this question are you sure your are competent to carry out inspections. ????????

I'm still trying to work out what he's actually asking?? lol!!

 

[Paultrim123321]

Sorry for the confusion, basically 25mm sub mains supply to building. 100A BS-88 protection at source.
In building the 25mm terminates into a non fused isolator where it splits to smaller 4mm/6mm & two 16mm single phase sub feeds to local fused isolators and two 9-way consumer units within the building. The fused isolators are rated at 20A & 40A and the CU's taking diversity into account demand approx 30A each.
My question is, should there be fuse protection where supply cables reduce from 25 or am I right in thinking reg 433.3.1 is applicable?

 

[richy3333]

Still not really sure what you're saying? Are you suggesting the 100A BS88 is protecting 4,6,16mm cables or am I just too tired to read?

 

[ackbarthestar]

Quite easy to suggest the appropriate improvement.
It must be your call, though, otherwise we also carry the liability and, of course, our commission.

What coding would you suggest?

 

[Paultrim123321]

Yes sorry its been a long week! My explanation is a bit complicated but basically the 4/6/16mm tails are protected by the 100A fuses at the source. the supply from source enters the building as a 25mm SWA then reduces.
The load on these cables though is protected by local fuses which are fine for the 4/6/16mm tails.

IMHO I am swaying toward C3 with a recommendation for future local protection to be provided to the tails if the installation undergoes any further alteration. its a small gymnasium and so we will be inspecting it every three years and we carry out other inspection work annually so we can at least keep up with any works.

 

[ackbarthestar]

Yes sorry its been a long week! My explanation is a bit complicated but basically the 4/6/16mm tails are protected by the 100A fuses at the source. the supply from source enters the building as a 25mm SWA then reduces.
The load on these cables though is protected by local fuses which are fine for the 4/6/16mm tails.

IMHO I am swaying toward C3 with a recommendation for future local protection to be provided to the tails if the installation undergoes any further alteration. its a small gymnasium and so we will be inspecting it every three years and we carry out other inspection work annually so we can at least keep up with any works.

So everything is protected then?

 

[Sintra]

The 4, 6 and 16mm tails are protected for overload even though the protection is upstream so no code IMO. The title of your thread is FAULT PROTECTION but what you have explained points more to overload protection. Can you elaborate a bit more.

 

[Paultrim123321]

The 20A fuses are at the load end of the tails. From the fuses are final circuits, for example Shower & a Sunbed. The tails are backed up by 100A fuses, final ccts protected by 20A fuses. Tails wired in 4mm/6mm. Final ccts wired in 4mm T+E.

Wish I had a photo to show the layout.

 

[Paultrim123321]

Your right about the overload protection statement, that's how I should have worded it. That's where I am referring to reg 433.3.1

 

[Sintra]

Think about it this way you have a circuit with a fixed load protected by a 20A protective device. This 20A protective device is fed with 4mm tails from a 100A fuse. How can the 4mm tails get overloaded?

 

[ackbarthestar]

So what's wrong with overload protection at the load end 433.3?

What you seem to worried about is S/C protection of the tails. There's always 434.2

 

[micknew]

If you are asking this question are you sure your are competent to carry out inspections. ????????

He never answered your question Dizzy wonder why, not competent or qualified?

 

[Paultrim123321]

Because he was holding my megger

 

[GLENNSPARK]

hmm...

 

[ackbarthestar]

hmm...

View attachment 20088

Now, don't tell me that's a self portrait ......?

 

[GLENNSPARK]

Now, don't tell me that's a self portrait ......?

oh i can do the face....
when i sees something being done i dont like....

 

[ackbarthestar]

oh i can do the face....
when i sees something being done i dont like....

There's a lot of things we don't like but are often covered by the BS.

Tell me about the garbage called manufactured electrical products foisted on us by wholeslers and trade counters

 

[Paultrim123321]

In all seriousness I'm not on here to get involved in the competent/non competent persons debate, I'm a little old in the tooth for that.

I am after some opinion on an installation I have come across this week that I thought I would share with you(rather badly I admit) and see what other people's thoughts are. Don't need any sarcastic remarks please