Breaking capacity question

[sparksburnout]

Apologies if this should be in a beginners section, but I can't seem to see one. Anyway here goes - I am having a bit of a mental battle regarding PSC and the breaking capacity of the protective device. Let's take a domestic setup as an example, with an 80A DNO fuse, BS1831 I think off top of my head. Now, I understand that under short circuit conditions (say if the installation side of the main CU switch was shorted), there is going to be a very large current flow, and that the switch or other device must be capable of withstanding it without exploding etc. I also understand that before the DNO fuse blows, there will be a current flow of many many times it's rating, for a short period of time going through both it and the other devices involved. Am I correct in thinking then, that before that particular fuse blows there will actually be large enough currents flowing for long enough to potentially destroy the other devices involved and lead to other danger? Surely the fuse will go before this happens? I guess not but I am just having difficulty getting my head round it.......

 

[jackhammerJIM]

BS 1361

this is why we measure the pscc .

 

[D Skelton]

Apologies if this should be in a beginners section, but I can't seem to see one. Anyway here goes - I am having a bit of a mental battle regarding PSC and the breaking capacity of the protective device. Let's take a domestic setup as an example, with an 80A DNO fuse, BS1831 I think off top of my head. Now, I understand that under short circuit conditions (say if the installation side of the main CU switch was shorted), there is going to be a very large current flow, and that the switch or other device must be capable of withstanding it without exploding etc. I also understand that before the DNO fuse blows, there will be a current flow of many many times it's rating, for a short period of time going through both it and the other devices involved. Am I correct in thinking then, that before that particular fuse blows there will actually be large enough currents flowing for long enough to potentially destroy the other devices involved and lead to other danger? Surely the fuse will go before this happens? I guess not but I am just having difficulty getting my head round it.......

A domestic set up is very unlikely to have a prospective short circuit current higher than most 6kA MCBs. I have only come across a high prospective short circuit current in a domesic premesis once, from memory it was around 7kA, the TX was next door. In this case I just installed commercial MCBs with an Ics of 7.5kA.

In commercial setups with very high prospective fault currents, the installation of an upstream device with higher breaking capacities (16kA, 33kA etc...) protect the installation and the lower rated devices downstream (6kA, 10kA etc...) because if these fail to operate, the fuses upstream will.

Remember though, if a fault of high enough magnitude takes out a HRC fuse before an MCB, your MCB is as good as dead and will need replacing.

The best advice all round is to spec devices with a high enough breaking capacity to deal with all levels of prospective fault current. Most decent manufacturers produce MCBs with high breaking capacities of between 10-20kA, some go up as high as 50kA.

With regards to your DB main switch, this will be rated at no less than 15kA which is what BS EN 60947 calls for.

 

[GLENNSPARK]

A domestic set up is very unlikely to have a prospective short circuit current higher than most 6kA MCBs. I have only come across a high prospective short circuit current in a domesic premesis once, from memory it was around 7kA, the TX was next door. In this case I just installed commercial MCBs with an Ics of 7.5kA.

In commercial setups with very high prospective fault currents, the installation of an upstream device with higher breaking capacities (16kA, 33kA etc...) protect the installation and the lower rated devices downstream (6kA, 10kA etc...) because if these fail to operate, the fuses upstream will.

Remember though, if a fault of high enough magnitude takes out a HRC fuse before an MCB, your MCB is as good as dead and will need replacing.

The best advice all round is to spec devices with a high enough breaking capacity to deal with all levels of prospective fault current. Most decent manufacturers produce MCBs with high breaking capacities of between 10-20kA, some go up as high as 50kA.

With regards to your DB main switch, this will be rated at no less than 15kA which is what BS EN 60947 calls for.

depends on the distance from the TX...

 

[D Skelton]

depends on the distance from the TX...

You've commented after only reading the first line again haven't you Glenn?

 

[ipf]

You've commented after only reading the first line again haven't you Glenn?

In the thread 'next door', Glenn ?

 

[Dizzy_Maskell]

Install an MCCB board in all domestic installations schneider NSX range are good for this usually rated around 25Ka.
I know it costs a little more than a simple consumer unit but it looks great and due to the size of then u can store all kinds of thinks on top of them

 

[SPARTYKUS]

A domestic set up is very unlikely to have a prospective short circuit current higher than most 6kA MCBs. I have only come across a high prospective short circuit current in a domesic premesis once, from memory it was around 7kA, the TX was next door. In this case I just installed commercial MCBs with an Ics of 7.5kA.

In commercial setups with very high prospective fault currents, the installation of an upstream device with higher breaking capacities (16kA, 33kA etc...) protect the installation and the lower rated devices downstream (6kA, 10kA etc...) because if these fail to operate, the fuses upstream will.

Remember though, if a fault of high enough magnitude takes out a HRC fuse before an MCB, your MCB is as good as dead and will need replacing.

The best advice all round is to spec devices with a high enough breaking capacity to deal with all levels of prospective fault current. Most decent manufacturers produce MCBs with high breaking capacities of between 10-20kA, some go up as high as 50kA.

With regards to your DB main switch, this will be rated at no less than 15kA which is what BS EN 60947 calls for.

Had a similar, mft came back with 6.9kA, scratched head, went outside, Transformer was 3 feet away other side of fence

 

[Engineer54]

Had a similar, mft came back with 6.9kA, scratched head, went outside, Transformer was 3 feet away other side of fence

Must have been a bloody small distribution TX if you were only reading 6.9KA!!

There are parts of the City of London and in Central London, where where you wouldn't see less than 10-12KA... Unless things have changed very drastically in latter years....

 

[GLENNSPARK]

You've commented after only reading the first line again haven't you Glenn?

hmmph...

 

[D Skelton]

Must have been a bloody small distribution TX if you were only reading 6.9KA!!

There are parts of the City of London and in Central London, where where you wouldn't see less than 10-12KA... Unless things have changed very drastically in latter years....

Funny you should say that, a mate of mine tells the story of when he was testing a flat in the city and was stumped as to why he was reading 20 odd kA. Turned out the TX was directly below the flat.

That said, it is not the norm in a domestic situation to get readings above 2-3kA.

 

[Engineer54]

In the parts of London that i specified above, relates to the density and reinforcement of the distribution systems. In some areas it was quite normal to measure 16KA, and would be confirmed by the then LEB. No idea how, or if things have changed since those times. Probably nothing much has changed, as the Privatised companies don't really like investing too much in the infrastructure unless they literally have too... lol!!

 

[SPARTYKUS]

Must have been a bloody small distribution TX if you were only reading 6.9KA!!

There are parts of the City of London and in Central London, where where you wouldn't see less than 10-12KA... Unless things have changed very drastically in latter years....

I'll take a picture its 4 miles away