higher PFC than MCB rating.

[monkey5]

just added a few sockets on a ring circuit and came to testing,

Ze - 0.05

PFC - 4.2kA

its an old wylex board with BS 3871 mcbs (i think these are rated at 6kA)in but a couple have been changed to 60898 but these are rated a 3kA can you get these rated with a higher kA or shall i source some of the 3871's

cheers wayne

 

[Andy™]

the service fuse will be rated to at least 16.5KA if BS1361. if its BS88, could up 80KA

 

[monkey5]

[quote name='Andy

 

[Andy™]

as a backup, yes.

as long as there is something that is capable of breaking PFC, then other devices can be rated lower

look at 3036 fuses - generally 1KA rated. and you often get over 1KA PFC

 

[monkey5]

[quote name='Andy

 

[septiclecky]

[quote name='Andy

 

[Andy™]

yes, an MCB breaking a value at or near its max PFC may not survive to be used again

 

[septiclecky]

[quote name='Andy

 

[sparkytim]

Andyas a backup, yes.

as long as there is something that is capable of breaking PFC, then other devices can be rated lower

look at 3036 fuses - generally 1KA rated. and you often get over 1KA PFC

Andy why would you presume back up protection is afforded to the downstream device ?

Just because the upstream device may handle 33kA doesn't mean that back up protection is afforded.

Regards chris

BRB Pg 76 434.5.1

 

[binky]

[quote name='Andy

 

[Andy™]

not right on which part?

 

[binky]

[quote name='Andy

 

[steptoe]

Im lost here,

someone has said that you can have a fault between 3 and 16.5 kA, if it is over the A rating of your protective device then the mcb/fuse etc will blow/trip,

if the fault is so great that it causes the protective device to ionise(is that the right word?)

then the main fuse will blow, on TNCS DNO state that it will not be greater than 16.5kA, and in general mthe main fuse is <80A,

so there we go, a fault at more than 1kA, which will cause the 3036 to ionise(there we go again with the wrong term) will definately blow the 80A incomer, long before 1kA is reached.

so what is the point trying to be made here?

I once asked a very similar Q on here, and now Im educated as to why it works.

your feet are for dancing.....

headbang

 

[binky]

your feet are for dancing.....

I use mine to insert in mouth.... ROTFWL

 

[Sidewinder]

plumber,

You also need to look at arcing and pre-arcing times as well as I^2t, energy let through, and the fault limiting of the incoming supply.

Also the downstream protective device does not need to be re-useable after a fault, a fuse is not, and an mcb is only rated for a small number of full current breaks.

I do agree that the system needs to be correctly assessed for breaking capacities, and this should be done during the discrimination calculations at the design phase as the two are somewhat linked.

The other issue with energy let through is it is manufacturers data and post install the designer is not really in control of this unless the install is under close management by competent persons and the H&S file required under CDM and the O&M manuals often contractually required, which can be inferred under CDM for safe operation and maintenance only specify certain brands of protective device then you have some control over the future life of the install.

Essay ends, sorry...

:coat

 

[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?

 

[binky]

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?

Good question, think the official answer might be yes, but I've never come across the aftermath of a main fuse blow. Would supect CU might be smoldering anyway

 

[Sidewinder]

steps,

I would say no you should not need to change all of them, but, you may need to change the one that carried the fault current, and depending on the condition of that device possibly those adjacent!!!

 

[steptoe]

Good question, think the official answer might be yes, but I've never come across the aftermath of a main fuse blow. Would supect CU might be smoldering anyway

and if you are working on it at the time then you would need to visit our new boots thread,

they may need replacing too,

and perhaps new trousers as well.

we used to have a saying,

ring an ambulance,

and tell them to bring a spoon/shovel*

(*delete as appropriate as to your toilet use/food consumption previously)

 

[Sidewinder]

Plumber,

I'm not clever enough with the forum software to requote again the message! :_|

I believe that the two are inextricably linked. :innocent

The behaviour of the device at the onset of failure has an affect on the physics of the final failure.

With regard to the incoming supply, it is not just the 1361 that needs to be considered, it is the behaviour of the total incoming supply loop, the DNO may quote max pfc figures, the Ze may give a definitive d.c. value, however, the dynamic fault current in actual fault conditions may be limited further by the DNO design, which could actually further limit the pfc.

As long as the upstream device interrupts the fault current then the down stream device is protected.

The thing is it is not as simple as just doing a bit of maths is it as you well know!

There are i's or j's involved depending on your schooling!

There are also a multitude of other factors, bordering on almost true chaos theory with todays modern installations and loads is there not?

I am guessing you have srudied chaos theory? :innocent

In your reply to steps you give some examples however, the nature of the fault current, would also need to be considered would it not, remember CIVIL?

:Blushing