higher PFC than MCB rating.

[Riggy]

Did anyone actually answer the OP's question?

 

[Andy™]

post 2

 

[Riggy]

[quote name='Andy

 

[Sidewinder]

Andy,

Sorry I don't think your post #2 did answer the OP.

Never mind, it has brought up a lot of discussion which is good!

Re-reading the OP mind I think he has god his 3871 & 60898's the wrong way around.

3871 typically M3 - 3kA, 60898's normally 6kA minimum unless we are talking about the plug in type to replace 3036's and I can't remember the ratings of these?

 

[Andy™]

re-reading, i agree he may have got the 3871 & 60898 wrong way. i know you can get 6KA 3871, but i cant remember any 3KA 60898

 

[wozz]

i think plug in 60898s are 3kA, too dark to go garage to check though.

just found

http://www.diy.com/diy/jsp/bq/nav.jsp?action=detail&fh_secondid=9289964&ecamp=trf-005&CAWELAID=266916980

 

[Andy™]

for some reason i thought the 60898 of them was 6KA

 

[Sidewinder]

Like the ABB moulded into the side of the Wylex breaker and the reference to 60439 in the text for a 60898 breaker, they don't even know what they are selling!

 

[davetheglitz]

Thought you might find this useful http://www.talk.electricianforum.co.uk/showthread.php?t=590

If you had a re-wirable fuse fitted a fault current of >1kA can cause the fuse to go explosively. So if the upstream fuse can take a fault current of >16.5kA I would assume that the downstream re-wireable fuse would be blown to bits with a fault of 3 or 4kA.

The other question is how likely is this to happen? To cause this the fault must occur on one circuit pretty damn close to the fuse board to ensure there is a low enough R1 + R2 or R1 + Rn to allow such a large current to flow anyway. I'd be interested to know if anyone has ever seen the effect - and more particularly ever seen it in a domestic environment.

 

[steptoe]

My redYou also need to look at arcing and pre-arcing times as well as I^2t, energy let through,

Sidewinder, For backup protection why would i need to consider the pre arcing, were not talking about discrimination?

and the fault limiting of the incoming supply.

as i said the total let through of the 1361 would need to be compared to the rating of the mcb(icu)

Also the downstream protective device does not need to be re-useable after a fault,

I stated the icu of the mcb This is the maximum it can break safely although it may no longer function

Steptoe

if we have a fault of 5000A then the mcb is effectively going to weld itself closed,

but the BS1361 will blow, fault >80A, but <33000A .

if the main fuse blows then we are going to change all secondary protective devices as a matter of course,

would that make sense?

Many fuses and mcbs offer energy limiting properties, especially on high fault currents, the fuses will break of the first half of the fault cycle. Take for example a 1361 type ii busman lr85 if we have a pfc of 33ka the fuse limits the let through to just over 10ka so if the downstream fuses were rated to say 15ka then backup protection has been afforded.

regards chris

plumber,

Im not about to get into the dazzle em with brilliance

or

baffle em with bull**** arguement

but

facts are facts

DNO states that under normal fault conditions(as under normal conditions we work to under EAWR and[shock horror] BS7671)

BS1361 and BS88 will accommodate any fault DNO import to us,

we are unable to generate a fault giving a current of (in excess) the primary protective device(ie <=100A as stated on the cut out[ive never seen one greater]) , so it is a DNO issue to protect against one greater, by which they do by stating they will never generate a fault greater than 16.5kA

any fault causing the cut out fuse to blow requires the attendance of a competence person to do an inspection and test, fault sorted and required parts changed.

case closed.

IMHO would be all protective devices changed, but various people have differing opinions to this, I dont argue this point,

I disagree with the fact that the ultimate protective device(DNO cut out) is inadequate, as you state, after all, its just another fuse.

yes, it may not be a fuse, and may well be a copper strip, DNO problem, and like so many other things, how do you know your 6A MCB will actually trip out at 7A.?

 

[steptoe]

Thought you might find this useful http://www.talk.electricianforum.co.uk/showthread.php?t=590 If you had a re-wirable fuse fitted a fault current of >1kA can cause the fuse to go explosively. So if the upstream fuse can take a fault current of >16.5kA I would assume that the downstream re-wireable fuse would be blown to bits with a fault of 3 or 4kA.

The other question is how likely is this to happen? To cause this the fault must occur on one circuit pretty damn close to the fuse board to ensure there is a low enough R1 + R2 or R1 + Rn to allow such a large current to flow anyway. I'd be interested to know if anyone has ever seen the effect - and more particularly ever seen it in a domestic environment.

what happens is the fuse ionises(Im sure thats not the right term but no one has said so yet)

that means it actually attracts the particles from the air, as it gets hotter it actually grows in size, meaning it can actually carry more current, ad infinitum.

NO, never seen in domestic, probably due to DNO fuse blowing

 

[kme]

Albert:

I think you wanted vaporise mate.

Ionising is somewhat different

 

[monkey5]

Andy,Sorry I don't think your post #2 did answer the OP.

Never mind, it has brought up a lot of discussion which is good!

Re-reading the OP mind I think he has god his 3871 & 60898's the wrong way around.

3871 typically M3 - 3kA, 60898's normally 6kA minimum unless we are talking about the plug in type to replace 3036's and I can't remember the ratings of these?

defo not got the mcbs wrong way round the 3871 are m6 the 60898 is a replacement for the 2871 and is only rated at 3kA.

cheers wayne

 

[Sidewinder]

KME,

What steps is describing is a chain ionisation reaction though, it is the formation of a plasma ball where by the air actually conducts, and the explosion can grow out o all proportion.

This is normally limited by the available current and the arc voltage though.

BTW, been away?

 

[wozz]

i thought step had it right, type in 'jacobs ladder' on you tube i believe that to be ionization

(yes sometimes i get very bored)

 

[steptoe]

Albert:I think you wanted vaporise mate.

Ionising is somewhat different

deffo the opposite from vaporise

KME,What steps is describing is a chain ionisation reaction though, it is the formation of a plasma ball where by the air actually conducts, and the explosion can grow out o all proportion.

This is normally limited by the available current and the arc voltage though.

BTW, been away?

thats it

i thought step had it right, type in 'jacobs ladder' on you tube i believe that to be ionization (yes sometimes i get very bored)

thank you

 

[kme]

Oops. I allus thought the plasma vaporised the item in question. My bad.

SW - Nah - just not too well ATM mate, thanks

Still awaiting info for G`einon.

KME

 

[binky]

1, we make sure the downstream mcb has a rating greater than the pfc

regards chris

or PSCC of course.

This is what I was taught many years ago, but lecturers have been known to be wrong .

 

[binky]

Right, got some time today so phoned NICEIC helpline.

In short, for most domestic situations, the service cut-out fuse protects against PFC/PSCC, so as long as that is less than 16Ka for standard 1361 fuse then CU is protected.

As far as MCBs within CU are concerned they are unlikely to see PFCs downstream of them that they can't handle because they only pass 6A or 32A. The only sceario that might overload MCB/fuse would be dead short at the CU if you touched a very short piece of wire to busbar or something (which is the scenario DavetheGlitz suggested), but even then the 1361 fuse is the protective device, and is highly unlikey.

The only time downstream PFCs becomes an issue is on large commercial installations where it is customary to measure PFC at diferent points of the circuits, a scenario with which I am not familiar because I don't do this type of work, so perhaps some of you industrial guys could further enlighten us on this.

So it would seem Andy was right, and my college lecturer was wrong :innocent

 

[user 6351]

If it helps, I got a non-conformance on my latest assessment because of this... the house that I used as my example is about 100 yds from the the substation, so I was getting PFC of 22k (ouch!!).

I'd fitted a bog standard board with 60898s in it, so was told that I would either need to upgrade to a board with fuses of a higher rating, or install a sub main to the other side of the house to get lots of resistance in and reduce the PFC to a more suitable level.

However, after a lot of head scratching (and stress about the expense I'd have to go to!!) the nice tech chap at Elecsa referred me to page 56 (I think, will check in a bit) of the OSG, which states very succinctly that in a domestic situation, the distributors fuse is perfectly acceptable for providing protection to any fault above that which the MCBs are rated at and that therefore, 60898s can be used.

A rather large sigh of relief followed...