Prospective fault current

[PrinceofDarkness]

What are you saying there, 123: That a supplier of a commercial or indusrtrial unit wouldn't cooordinate the sevice cut-out, to suit the characteristics of his supply?

I spent many years with the LEB, so I know that that isn't the case.




Lucia.

 

[Spark123]

If you fired 30KA through a 6KA MCB then I wouldn't like to be around to see the consequences!
30KA is a typical figure if you are in a substation with a 1MVA transformer.

 

[Spark123]

What are you saying there, 123: That a supplier of a commercial or indusrtrial unit wouldn't cooordinate the sevice cut-out, to suit the characteristics of his supply?

I spent many years with the LEB, so I know that that isn't the case.




Lucia.

Some commercial or industrial sites get their electricity at 11kv, 33kv, 132kv etc and use their own transformers. The correct choice of fuses is then up to them.
Where I work we don't have a service cut out.
Also, if that was true why are MCBs available in higher KA ratings, for example Square D offer SQO range 6KA and KQ range 10KA.
Wylex do NSB iirc 6KA and PSB iirc 10KA too.

 

[Lectrician]

If you fired 30KA through a 6KA MCB then I wouldn't like to be around to see the consequences!
30KA is a typical figure if you are in a substation with a 1MVA transformer.

Yes, but you co-ordinate your fuse or MCCB before this MCB. Think about your 'let through energy'. It maybe 30ka at the origin, but you will have a fuse or MCCB before your MCB's. If you do not, you have not designed your install correctly.

Some commercial or industrial sites get their electricity at 11kv, 33kv, 132kv etc and use their own transformers. The correct choice of fuses is then up to them.
Where I work we don't have a service cut out.
Also, if that was true why are MCBs available in higher KA ratings, for example Square D offer SQO range 6KA and KQ range 10KA.
Wylex do NSB iirc 6KA and PSB iirc 10KA too.

Again, think about let through energy. Based on this you can decide if you need 6 or 10kA.

Having an MCB connected directly to the TX with no intermediate fuse or MCCB would not be any good.

 

[PrinceofDarkness]

OK, 123. I don't want to fall out with you, particularly since you've widened the topic to supplies of 132kV. That wasn't the kind of supply that I had in mind, and I doubt that many here have to deal with such lofty issues.

Regardless of the supply, whether LV or HV, there's no such thing as not having some sort of cut-out or disconnector for the intake to separate it from the grid.



Lucia.

 

[Spark123]

Some commercial or industrial sites get their electricity at 11kv, 33kv, 132kv etc and use their own transformers. The correct choice of fuses is then up to them.
Where I work we don't have a service cut out.
Also, if that was true why are MCBs available in higher KA ratings, for example Square D offer SQO range 6KA and KQ range 10KA.
Wylex do NSB iirc 6KA and PSB iirc 10KA too.

Again, think about let through energy. Based on this you can decide if you need 6 or 10kA.

Having an MCB connected directly to the TX with no intermediate fuse or MCCB would not be any good.

Nope, they would go bang! Think I saw it on a training video a few years ago.
How do you decide wether a 6KA or a 10KA will suffice, for example if we had PFC of 30KA at the MCB board and upstream protection for the board was 100A BS88 fuse links?
Is there another value on the MCB, such as a withstand value? I just don't see how you can design backup protection with only knowing a let through "energy" value and a max breaking "current" value.

 

[Lectrician]

You need the manufacturers data.

 

[PrinceofDarkness]

Spark 123, continuing with your hypothetical situation (and putting your 'training video' to one side): So you have a PSCC of 30kA. How would that help you to decide whether to select a 6kA or a 10kA device?

The PSCC is dealt with by the service fuse. Any other selection of MCB rating or type should be coordinated to its load and not to the supply characteristics.......



Lucia.

 

[Electrifying]

The PSCC is dealt with by the service fuse. Any other selection of MCB rating or type should be coordinated to its load and not to the supply characteristics.......

Lucia.

Once again - not following this line of thinking.

If the PSCC was dealt with by the service fuse, why doesn't it blow on a short circuit everytime?

It doesn't - the MCB trips out - otherwise we'd be forever popping down the wholesalers for 100 A fuses.

I think the word is discrimination - the downstream device will trip first.

It stands to reason, then, that the MCB needs to be able to carry the maximum fault current, even if this will never flow due to the current limiting properties of the device.

If what you say above is true, then explain one thing - why do they bother printing the two figures on an MCB?? - just for the hell of it??

 

[PrinceofDarkness]

You worry me, Electrifying. The fact that you can't differentiate between a normal short-circuit on a standard circuit, handled by the MCB, and the PSCC of the supply. I've already given the definition (as per the ESI) of PSCC. But, if you're still confused, I have the patience to explain it again, for your personal benefit.


Lucia.

 

[Electrifying]

You worry me, Electrifying. The fact that you can't differentiate between a normal short-circuit on a standard circuit, handled by the MCB, and the PSCC of the supply. I've already given the definition (as per the ESI) of PSCC. But, if you're still confused, I have the patience to explain it again, for your personal benefit.


Lucia.

I'm all ears - coz if you're trying to explain to me that the fault current that will flow at the main switch incomming tails is going to be thousands of amps bigger than the fault current that will flow at a point 2 inches the other side of a 32 A MCB, I can't wait.


Oh yes, please answer my question from my previous post within your explanation - this should be fun!!

 

[Spark123]

Depending where the fault is, the impedance of the circuit will go towards limiting the fault current.
You can use the cutout fuse as backup protection in almost all cases yes, but not in all cases.
There has got to be some relationship somewhere between the energy let through of the device and the withstand value, as lec says you'd need to consult manufacturers data. I'd expect the 10kA to have a higher withstand value than the 6kA, however we just tend to use BS88 fuses

 

[Electrifying]

Depending where the fault is, the impedance of the circuit will go towards limiting the fault current.
You can use the cutout fuse as backup protection in almost all cases yes, but not in all cases.
There has got to be some relationship somewhere between the energy let through of the device and the withstand value, as lec says you'd need to consult manufacturers data. I'd expect the 10kA to have a higher withstand value than the 6kA, however we just tend to use BS88 fuses

Hi Spark123,

I'm finding it difficult to tell which side of the argument you are on

If the fault occurs within the CU - say the line conductor shorts out to the neutral bar - then the fault current is not gonna be much different from the fault current measured at the tails.

As far as I'm concerned, the 'manufacturers data is written on the MCB - a max fault current and a serviceable fault current.
You measure the PFC of an installation to confirm that it is lower than the breaking capacity of your devices -

You could, if you liked, measure the PFC at every device - as that's the one that matters to each device - but that could be time consuming on an installation with multiple 32way boards, so the PFC at the origin (being the maximum that could flow) can be used to compare to your devices.

It ain't the rocket science that some people are trying to lead everyone to believe

 

[Spark123]

You could have a fault in the CU and blow the service fuse if the current gets high enough for long enough.
It is just more normal to have a fault down the cable a bit which introduces impedance thus limiting the fault current down so only the circuit protective device operates.
I don't think they print the servicable fault current on the MCB, the one in the box is just the maximum.

It is plausable to fit a board with a lower max fault current providing that backup protection is provided.

 

[Electrifying]

You could have a fault in the CU and blow the service fuse if the current gets high enough for long enough.
It is just more normal to have a fault down the cable a bit which introduces impedance thus limiting the fault current down so only the circuit protective device operates.
I don't think they print the servicable fault current on the MCB, the one in the box is just the maximum.

It is plausable to fit a board with a lower max fault current providing that backup protection is provided.

The full figure e.g. 6000 is in Amps and is the maximum.

The single figure - normally directly below e.g. 3 is in KA and is the serviceable figure.

Untitled

• Electrifying
• 13 Apr 2010
• 1