protecting cable inside a distribution panel.

[Inteificio]

Hi,

Just came across something earlier that made the have a think.

Distribution panel fed by 25mm cable.
This goes directly in to a D50 breaker.
The D50 feeds 4 6mm cables squeezed in top of the MCB that feeds small 3 phase bus bars, these in turn feed 1.5mm circuits.

I have fitted MCBs to cover the 1.5 cables, and was about to exchange the D50 for an RCD.

This made me think.
If I remove that D50 nothing is protecting the 6mm cables or the bus bar (10 mm) from overload.
If all of the circuits are in use then those 6mm wires will cook.

However no distribution board I have worked on has provision for this.
Discrimination lets us build the board, but what protects the board if it is overloaded?

It seems such a basic thing, but I asked all the sparkies on site. The response from all was similar, "good point, no idea what the regs for it are"

So do any of you guys know what the requirements are to protect bus bars/internal cables from overload?

 

[D Skelton]

Correct design is what protects the main panel and tails from overload. You design a system working out its max demand, the tails are sized accordingly and the board specced to this.

 

[Pete999]

Confused got any pics, the description is a bit unclear

 

[Inteificio]

No pics sorry.

To make the question more general.

On all industrial distribution boards, the sum of all the breakers fitted will exceed the carrying capacity of the cable feeding the breakers.

So say you have a 6mm cable feeding the MCBs, and have 125A worth of MCBs. If the install is being used as designed then the 6mm will be fine, but what about when it is overloaded?

My board has a D50 instead of an incomer, therefore has protection that is not normally on a board. That protection is insufficient, but is more than is normally given.

It's a weird one.

 

[Tiny Spark]

Ime still confused....

Your board will have a incooming breaker of size X (All of ours are 125A) That will feed internal bus bars which will be rated for current X and some.
Those bus bars then feed the outgoing breakers which your circuits connect to.

Dont see the issue??

EDIT:

Wait a second are you saying that you have 6mm cable connecting the D50 to the bus bars??

 

[D Skelton]

On all industrial distribution boards, the sum of all the breakers fitted will exceed the carrying capacity of the cable feeding the breakers.

The sum of all your breakers is not your maximum demand.

So say you have a 6mm cable feeding the MCBs, and have 125A worth of MCBs. If the install is being used as designed then the 6mm will be fine, but what about when it is overloaded?

If it is even possible to overload the 6mm cable then the installation has not been designed correctly.

 

[Inteificio]

EDIT:

Wait a second are you saying that you have 6mm cable connecting the D50 to the bus bars??

yes I am.

 

[Tiny Spark]

Well I dont have a regs book to hand, but ime sure someone will and be able to enlighten us but I think it should be at least 10mm.

I think 6mm is only good for about 40-45A?

 

[Inteificio]

It is coming out, so have not checked, but am hoping it is XPLe which is good for 56.

 

[Tiny Spark]

It is coming out, so have not checked, but am hoping it is XPLe which is good for 56.

Should be fine then.

 

[Darkwood]

Submain - supply cables should be protected from overload like any other final circuit, on some occasions you can fit smaller tails to a dist-board from the DNO supply if the nature of the dist' board won't permit the tails to be overloaded ... E.G. a 1way shower board can be tailed off the DNO supply with 10mm as the OCPD for the shower will never permit its CCC to be exceeded... i still laugh when i see how someone has struggled to get 25mm tails into a 1way board lol..

 

[SPARTYKUS]

In free air too so top of the list in Amperes

 

[Des 56]

Submain - supply cables should be protected from overload like any other final circuit, on some occasions you can fit smaller tails to a dist-board from the DNO supply if the nature of the dist' board won't permit the tails to be overloaded ... E.G. a 1way shower board can be tailed off the DNO supply with 10mm as the OCPD for the shower will never permit its CCC to be exceeded... i still laugh when i see how someone has struggled to get 25mm tails into a 1way board lol..

I take it since that experience you have used smaller tails

 

[Darkwood]

I take it since that experience you have used smaller tails

Had it been anyone else Des it would be plausible but I haven't been a victim to this situation as I don't really do domestic my learning curve was not picking tri-rated singles when wiring a 400amp busbar and and using every muscle I had to shape the stranded tails lol .... a long time ago though