High PFC. Cascading

[GEOFF S]

Careful there, it very much depends on how close the DB/CU is to the source of the supply. For arguments sake if a final circuit DB/CU is located within a main switchboard room next to the source TX, to say supply the lighting and small power circuits for the area, everything could change... We always install 22/25KA MCB's in such circumstances.

What you are describing in your 2nd and 3rd paragraphs above, is called a ''Protection Coordination Study'' and will cover both MV and LV protection devices from the TX/ Main switchboard all the way through to a sample final circuit on the furthest final circuit DB/CU. This is probably far beyond the scope of most electricians, but is common practice if not a requirement on the larger projects with extensive electrical distribution systems.

Not sure i follow, Annex Za is for Domestic single Phase 100 amp systems, along with other requirements, its not for comercial or industrial purposes, the theory is the same.

Well the study will require amongst other things, knowledge of symetrical components and other network theorms, this is degree level engineering, most use software.

It certainly is a requirement on all installations, the problem with large systems is the complextity of of high fault currents, assymetry, paralell and series motors,generator etc.

Cheers

 

[spark 68]

Yes annexe ZA refers to SP domestic type CUs, however if you look at the spec for say Schneider Isobar TP+N DBs they also have a similar higher "conditional rating" for the complete type tested assembly which is rather higher than the individual OCPDs, so there must be an equivalent annexe or similar for those too, which is hardly surprising as the same electrical theories apply to those too.

 

[GEOFF S]

Yes annexe ZA refers to SP domestic type CUs, however if you look at the spec for say Schneider Isobar TP+N DBs they also have a similar higher "conditional rating" for the complete type tested assembly which is rather higher than the individual OCPDs, so there must be an equivalent annexe or similar for those too, which is hardly surprising as the same electrical theories apply to those too.

I think what you are looking at is the withstand value, one part of the conditional rating for example will be the X/R ratio.

Cheers

 

[spark 68]

I think what you are looking at is the withstand value, one part of the conditional rating for example will be the X/R ratio.

Cheers

That is above my pay scale , I can only go from what Schneider technical told me, as I had a higher PFC reading at the intake on one of their DBs than the individual MCB/RCBOs allowed for on an EICR, on querying this with Schneider I was assured that the DB as a whole assembly was conditionally rated higher, this does not however take into account any back-up cascaded fault protection.

 

[GEOFF S]

That is above my pay scale , I can only go from what Schneider technical told me, as I had a higher PFC reading at the intake on one of their DBs than the individual MCB/RCBOs allowed for on an EICR, on querying this with Schneider I was assured that the DB as a whole assembly was conditionally rated higher, this does not however take into account any back-up cascaded fault protection.

What was the incommer? Its probable that the incommer provided that the condtions are met, will limit the peak current, allowing the MCB to be installed.

So, same theory.

Cheers

 

[spark 68]

What was the incommer? Its probable that the incommer provided that the condtions are met, will limit the peak current, allowing the MCB to be installed.

So, same theory.

Cheers

If I recall I had something around the 19kA mark (my tester does do Phase to phase measurement) , the MCBs were rated I think at 10kA.

 

[Engineer54]

Not sure i follow, Annex Za is for Domestic single Phase 100 amp systems, along with other requirements, its not for comercial or industrial purposes, the theory is the same.

Well the study will require amongst other things, knowledge of symetrical components and other network theorms, this is degree level engineering, most use software.

It certainly is a requirement on all installations, the problem with large systems is the complextity of of high fault currents, assymetry, paralell and series motors,generator etc.

Cheers

Well, i can still remember to this day of taking loop tests on a friends house that had a 1 MVA ''Eastern Electricity'' (shows you how long ago, lol) substation that actually made up as part of his boundary wall. The PFC measured was 17.8 KA, at his 80A service fuse and his MCB's being rated at just 4 KA (M4)... The substations TX had been uprated from 500 KVA a few years previously....

As i say, depends very much on just how close you are to the source of the supply, as to whether that 16KA rule stands or not...

The contractor or his designated design Engineering company uses software, but i still review/check submissions manually, especially where protection points are looking/getting too close...

 

[GEOFF S]

Well, i can still remember to this day of taking loop tests on a friends house that had a 1 MVA ''Eastern Electricity'' (shows you how long ago, lol) substation that actually made up as part of his boundary wall. The PFC measured was 17.8 KA, at his 80A service fuse and his MCB's being rated at just 4 KA (M4)... The substations TX had been uprated from 500 KVA a few years previously....

As i say, depends very much on just how close you are to the source of the supply, as to whether that 16KA rule stands or not...

The contractor or his designated design Engineering company uses software, but i still review/check submissions manually, especially where protection points are looking/getting too close...

Thats the problem with loop impedance testers, a 1MVA Tx, the max symetrical fault current 3 phase is around 32kA at a Z of 4.75%, so a single phase fault wil be half that, this is at the terminals of the Tx.

Now in reality it wont be an infinite bus, there will be a voltage depression on the primary, reducing further the fault current.

Now take into account the BS88 fuse limiting the current then if the CU meets annex ZA requirements, happy days.

Cheers

 

[Engineer54]

haha!! so not much point in any of these manufacturers producing 10, 22, 25 , ....50KA final circuit MCB's then, just rely on the upstream BS 88 Fuse, and it's all happy days !!!

 

[GEOFF S]

haha!! so not much point in any of these manufacturers producing 10, 22, 25 , ....50KA final circuit MCB's then, just rely on the upstream BS 88 Fuse, and it's all happy days !!!

Well, we are refering to domestic single phase installations, now if we move to three phase then the fault currents are much greater, couple this with motors and generators, derating due to poor PF, we then will need equipment with higher withstands. So they are required, just not for single phase domestic supplys, that is unless you are not installing to UK annex ZA.

Cheers


Cheers

 

[Engineer54]

From the very get go on this thread i have clearly stated that it is unwise to use a cover all statement in BS7671. I quite agree that in the vast majority of cases it can be used quite safely, on domestic or small commercial situations, but you need to think a little more, when a DNO substation is almost on the property. ...Well if you're wise you would!!