I need something explaining!

[ADS]

As the title says:

Could someone please explain to me, in really plain English, how the theory regarding 'let through' energy ratings of BS1361 main fuses, upstream of MCBs, affects the max PFC ratings of these MCBs.

I've read the technical data etc. and kind of get the gist of it - but I'm struggling to get my head around exactly what it is that is happening, and how it will limit the fault current through an MCB, because of the BS1361.

Am I missing the obvious or does this give other people a headache?

 

[spinlondon]

As I understand it, let through is the maximum amount of current that the fuse will allow through before the fuse blows.

From the table in Appendix 3, with an 80A fuse, the maximum it will let through before blowing is 1100A.

 

[ADS]

So, is it just a simple case of the BS1361 blowing before the max PFC is reached?

In other words (put simply):

If you had a PFC of 16 kA, you could actually use 6kA rated MCBs - as long as your upstream (back-up) protective device would interupt the fault current at less than 6 kA.

I know this is a simplified and general explanation, but is that the principle behind it?

If it is it kind of makes determining max PFC a bit pointless.

 

[Evans Electric]

I have read somewhere that you can rely on the main fuse to determine the rating of your MCBs . Don't ask me where .

 

[NozSpark]

The way I read it is that a device can only be guaranteed to operate within its ka rating, anything over it and it may not operate; hence having a main fuse 1361 type II allows a PFC of upto 16KA no matter what the KA rating of the circuit protective device.

I may be wrong though

 

[sparkytim]

I think it all comes down to speed of operation, i.e. the 1361 blows in a shorter time then it takes for the downstream MCBs to be damaged even if the fault is in excess of the MCBs PFC rating.

A 100 Amp 1361 Service Fuse is rated at 33 KA and is acceptable as back up protection for a 6 KA rated MCB

 

[philbas]

I have read somewhere that you can rely on the main fuse to determine the rating of your MCBs . Don't ask me where .

Where did you read this

 

[philbas]

I have read somewhere that you can rely on the main fuse to determine the rating of your MCBs . Don't ask me where .

Where did you read this

 

[sparkytim]

He did say "Dont ask me where".

I have read this too and have been searching for it.

NIC Inspection, Testing, Certifcation and Reporting book Pg 215 states

"The short-circuit capacity of a device must be no less then the PFC at the point in the circuit at which it is installed. The only exception to this rule is where a device having a lower short-circuit capacity is installed in conjunction with what is sometimes called "back-up" protection (for example a circuit-breaker backed up by an appropriate HBC fuse)"

 

[kme]

So.... the phrase "in conjunction with" then becomes subject to interpretation, surely?

Are we talking about two OCPDs in the same enclosure?, or just in the same installation? If the latter, then the KA rating of MCBs in a domestic environment becomes totally moot.

Now I`m getting confused, and I thought I understood this. Now I`m doubting meself....

I would suggest Sidewinder will be able to shed some light, as long as he remembers to turn "tech" into " semi-tech"... `cos thats the best I can manage........

n.b. " fish out of water" ATM, I`m in Dunstable Travel-lodge all week; for the MCS course. Ruddy useless place doesn`t even have a bar..........

(good job I picked up a pack of the Kronenbourgs on my way............)

Missing Mrs. KME

Missing me dog too

And the cat

And my office chair - this thing is playing carp with me back.........

 

[Badger]

So.... the phrase "in conjunction with" then becomes subject to interpretation, surely?Are we talking about two OCPDs in the same enclosure?, or just in the same installation? If the latter, then the KA rating of MCBs in a domestic environment becomes totally moot.

Now I`m getting confused, and I thought I understood this. Now I`m doubting meself....

I would suggest Sidewinder will be able to shed some light, as long as he remembers to turn "tech" into " semi-tech"... `cos thats the best I can manage........

n.b. " fish out of water" ATM, I`m in Dunstable Travel-lodge all week; for the MCS course. Ruddy useless place doesn`t even have a bar..........

(good job I picked up a pack of the Kronenbourgs on my way............)

Missing Mrs. KME

Missing me dog too

And the cat

And my office chair - this thing is playing carp with me back.........

argh but at least you have t'internet

 

[Sidewinder]

Mr. KME,

HOW MANY TIMES, its Premier Inn, Not bl**dy Travel Lodge IF you need a bar!

You should have asked me, I've probably stayed near there! ; )

K, I'll try to KISS.

Also, I'm a bit peeved that I get landed with this.

You for one know how much I miss "teaching/training" & how much I can't resist a challenge!

This however could go on for a while because, as Scotty says "Ye Cannae Deny The Laws Of Physics Captain".

1st we need to understand energy let through and what it is capable of.

One can release MW/MJ (Mega Watts/Joules) of "energy" over a long period and it has no destructive potential. We are all alive (I guess, if, we are reading this?) we will also have metabolised MJ (Mega Joules) of energy during our lifetimes from sugar, carbohydrates etc.

It has NOT caused us yet to explode in a ball of intestines I guess?

Thus the energy release if done in a controlled manner has no issues.

Think also about the energy consumed "normally" in an electrical installation. It causes no problems with destructive damage etc.

Next thing is to understand the relationship between MW & MJ etc.

Are we all OK with this so far?

p.s. I'm quote busy at the moment and may not be able to keep this up every night, sorry. I will however, if the interest is there try to explain it to the best of my ability as I understand it.

 

[Apache]

Are we all OK with this?

I am

 

[Sidewinder]

I am

WHY AM I NOT SURPRISED! O)

 

[spinlondon]

So, is it just a simple case of the BS1361 blowing before the max PFC is reached?In other words (put simply):

If you had a PFC of 16 kA, you could actually use 6kA rated MCBs - as long as your upstream (back-up) protective device would interupt the fault current at less than 6 kA.

I know this is a simplified and general explanation, but is that the principle behind it?

If it is it kind of makes determining max PFC a bit pointless.

Yes, in domestic situations there is very little point in determining PFC.

As it is generally accepted that any HRC fuse upto and including 100A, is sufficient to limit the current, without reliance on other measures.

It is in commercial/industrial systems where determining PFC is more relevant, as not only is it neccessary to ensure circuit protective devices are rated appropriately, but also descrimination between protective devices should also be assessed.

 

[Chr!s]

Let the Plumber Help:^O

 

[ADS]

Yes, in domestic situations there is very little point in determining PFC.As it is generally accepted that any HRC fuse upto and including 100A, is sufficient to limit the current, without reliance on other measures.

It is in commercial/industrial systems where determining PFC is more relevant, as not only is it neccessary to ensure circuit protective devices are rated appropriately, but also descrimination between protective devices should also be assessed.

Yes, I can see your point.

There will obviously be a point where current-limiting back-up protection will start to cause problems with discrimination between the devices.

I think this is the part that I was having trouble grasping.

 

[Chr!s]

100 amp 1361 type II fuse(33ka),

Pfc at origin 20Ka

Downstream fuse 32 amp Type b 60898 mcb(breaking capacity 10 KA and 6ka)

Total discrimination assumed

1361 Fuse(20ka)-----------------32 amp mcb(10ka)----------x fault(pfc 16ka)

1361 Fuse(20ka)-----------------32 amp mcb(6ka)-----------x fault(pfc 16ka)

Now the 1361 can break the 20ka safely, but as we can see the pfc downstream of the mcbs is greater than there breaking capacity.

So we look at the characteristic's of the 1361 fuse to see if back up protection(cascading) is afforded.

The pfc is limited by the fuse to around 9ka.

So from this we can see that the first example back up protection is afforded as the mcb has a rating greater than the 9ka.

In the second example the mcb has a rating of 6ka so back up protection is not afforded.

 

[ADS]

Hi Plumber,

Good explanation - but is this 'back-up' in case of failure of the MCB to open under fault conditions?

Because surely it will open well before the max PFC is reached, at which point that will be the end of the fault current anyway.

I am, of course, assuming that a 16 kA fault current doesn't just appear and actually builds up to the max - but either way, surely the MCB will clear the fault well before the maximun, without any need for the 1361 to blow?

 

[Lurch]

The pfc is limited by the fuse to around 9ka.So from this we can see that the first example back up protection is afforded as the mcb has a rating greater than the 9ka.

In the second example the mcb has a rating of 6ka so back up protection is not afforded.

Is that right, surely backup protection is afforded in both scenarios? Or maybe now we've gone all schoolesque some of the terminology has become a bit wacky!