Prospective fault current

[Electrifying]

Hi PrinceofDarkness

I do understand what you are saying but my argument is that as you are measuring your PFC at the incommers, there isn't anything of 'appreciable impedance' to affect your reading as any impedances are taken into account when your meter calculates the prospective fault current that could flow at that point.
And to be honest, an MCB is of negligible impedance, so even measuring with this in the circuit would give a reasonable idea of maximum pfc.

But the Maximum pfc is different to the 'actual' fault current, because, as Ericmark correctly pointed out, the protective device limits the fault current flow by operating fast enough to cut out before it reaches that level. But you still need to take the maximum into account in case it doesn't open fast enough and the full fault current flows.

I didn't mean to have a pop, it was more to do with Ericmark's patronising manner, implying that it shouldn't be posted on here because none of us will understand.
I've seen some c**p posted on that IET forum

 

[ban-all-sheds]

How is an MCB not 'of negligible impedance'??

It isn't, but that's not the point.

How is it limiting the current or having an 'appreciable impedance'
.
.
Oh Oh, where's our MCBs 'appreciable impedance' or current limting feature??

An MCB may limit the current which flows by interrupting it before it can peak.

So the definition of PFC has to be based on what the fault current would be if there was no protective device.

But you know that...

But the Maximum pfc is different to the 'actual' fault current, because, as Ericmark correctly pointed out, the protective device limits the fault current flow by operating fast enough to cut out before it reaches that level. But you still need to take the maximum into account in case it doesn't open fast enough and the full fault current flows.

 

[plugwash]

If your meter wont do phase to phase then you just double up the maximum phase to neutral value.

Be aware that while doubling up the maximum phase to neutral value gives a fairly conservative estimate of the phase-phase value it is technically possible for it to be higher. The most obvious scenario under which this would happen is a bad neutral.

 

[Electrifying]

An MCB may limit the current which flows by interrupting it before it can peak.

So the definition of PFC has to be based on what the fault current would be if there was no protective device.

Hi Ban,

As you spotted from my second post, I understand this bit - what I'm not now getting is the breaking capacity.

Why do MCBs have a breaking capacity of ,say, 5000, if they have a current limiting factor which won't let the PFC get anywhere near that anyway, regardless of what your measured value is?

 

[mfarrow]

MCBs aren't designed to limit fault current, only fuses and certain MCCBs can do that.

The two values you get on the side of an MCB are firstly the allowable fault current which will allow it to function again after a fault and secondly the allowable fault current which will mean it's days are numbered but it won't catch fire or explode in your face.

MCBs are slow devices. By the time an MCB has realised there's a fault, unlatched itself and broken the fault, the supply current has gone through zero several times.

A fuse on the other hand will see a fault current rising on the first cycle and safely break the fault before it's reached it's peak.

 

[mfarrow]

I've got a better picture somewhere, but roughly this is what the current curve looks like on fault for a fuse.

View media item 21350
Nobody's ever explained what happens if you make onto a fault mid-cycle, but I imagine it just means it will break at a higher peak current but within a shorter time (as I²t is the constant), so the same damage is done but no worse than if we start at zero.

 

[Electrifying]

MCBs aren't designed to limit fault current, only fuses and certain MCCBs can do that.


MCBs are slow devices. By the time an MCB has realised there's a fault, unlatched itself and broken the fault, the supply current has gone through zero several times.

Not according to this extract from a description of Wylex MCB operation

2 Short circuit conditions
When the current flowing through the MCB reaches a predetermined
level, the solenoid directly pulls in the plunger which forcibly separates
the contacts and simultaneously releases the trip mechanism.
3 Establishment of arc between fixed and moving contacts
As the moving contact moves away from the fixed contact, an arc is
established. The arc runs along the arc runner to the arc chamber
where it is split up between the plates and extinguished.
The low inertia and consequent high speed of the moving contact has a
limiting effect on the flow of fault current. The rapid development of
the arc, together with its accelerated extinction in the arc chamber,
gives a typical operating time of 3.5–5 milliseconds.
CURRENT LIMITING ACTION
The high speed current limiting action ensures that the MCB operates
before the full prospective fault current is allowed to develop.
Under fault conditions, damage can be sustained to the installation and
associated equipment due to the amount of energy that passes before
the current is completely interrupted. The total energy let-through
depends on the value of current and the time for which it flows, and is
denoted by the symbol I2t. The high speed current limiting action of
NS MCBs ensures that the energy let-through and any subsequent
damage is minimised. This reduced energy let-through assists greatly
with both back-up and discrimination considerations.


What I am asking, though, is.....If the MCB has a 'breaking capacity' of 6KA, does this mean it can take a flow of 6KA (ie after its current limitation) or does it mean that 6KA is the maximum fault current it can handle, then it limits the flow to, say, 2KA, by operating that quickly?

 

[mfarrow]

The rapid development of
the arc, together with its accelerated extinction in the arc chamber,
gives a typical operating time of 3.5–5 milliseconds.

Interesting, so they're not as dumb and slow as I thought

It'll still be slower than a fuse when it needs to be (i.e. high PFC which reaches large current values within 3.5ms), so you still need backup protection when this is the case.

What I am asking, though, is.....If the MCB has a 'breaking capacity' of 6KA, does this mean it can take a flow of 6KA (ie after its current limitation) or does it mean that 6KA is the maximum fault current it can handle, then it limits the flow to, say, 2KA, by operating that quickly?

It means it can break 6kA, i.e. above that the current will rise too quickly for the extinguisher and the contacts will just weld together.

 

[Electrifying]

It means it can break 6kA, i.e. above that the current will rise too quickly for the extinguisher and the contacts will just weld together.

That does make sense

 

[Jacobssi]

Ok, on subject of PFC and short circuit capacity, what has to be higher? My personal notes say a protective device short circuit capacity should be more than prospective fault current, are my notes correct? If so can someone please give me an example with fault figures. Thank you

 

[Spark123]

The short circuit capacity of the MCB needs to be higher than the prospective fault current at the point of installation.
If the PFC exceeds the short circuit capacity of the MCB it can fail to function or even explode

As someone mentioned before it is possible to exceed the value where backup protection is provided from an upstream device such as a BS88 fuse.

 

[PrinceofDarkness]

Much is made of the SC capacity of MCBs versus the measured (or enquired) value of the PSCC. But the fact is that the service fuse is there to handle such matters, so it doesn't matter a fig if the MCBs are M6 or M9 or whatever, in that respect.

If this were not the case, then the wholesaler would have to supply a range of MCBs for standard CU's to suit a particular installation..........



Lucia.

 

[Spark123]

In a domestic that may be true, however if you make the jump to commercial or industrial it certainly isn't.

 

[Jacobssi]

The short circuit capacity of the MCB needs to be higher than the prospective fault current at the point of installation.
If the PFC exceeds the short circuit capacity of the MCB it can fail to function or even explode

As someone mentioned before it is possible to exceed the value where backup protection is provided from an upstream device such as a BS88 fuse.

so it could explode due to more current than is designed to go through it? When designing a circuit does this have to be a limiting factor?

 

[Jacobssi]

The short circuit capacity of the MCB needs to be higher than the prospective fault current at the point of installation.
If the PFC exceeds the short circuit capacity of the MCB it can fail to function or even explode

As someone mentioned before it is possible to exceed the value where backup protection is provided from an upstream device such as a BS88 fuse.

so it could explode due to more current than is designed to go through it? When designing a circuit does this have to be a limiting factor?