PSC test problem

[lee0069]

Hi

I am currently working on a site and there main DB is like 3 meters away from there transformer.
The problem that i can see is that when i do a PSC test betwwen phase and nutural i get 0.01 ohms and a PSC of >10KA
then its X2 so 20KA arcoss the phases.

Now they have a DB with MCB`s in and some of them only have a 6KA rating on them. So i guess in the event of a short circuit they would explosde.

They do have 3 100amp HRC company fues but would these protect the MCB`s from exploding , and if yes if there ever was a short circuit would it just keep blowing the company fuses?

Thanks in advance

 

[Richard Burns]

It may be an idea to read this thread and see if you can get a more accurate reading of PSC, just in case your meter is slightly inaccurate at the low end of the range.

for remote DBs it is worth measuring the PSC at that point in case it is low enough to permit the 6kA MCBs.
Distribution boards are not specifically designed to ensure adequate containment in case of a short circuit greater than the breaking capacity (though they should be fairly safe).

You would now need to explore the current limiting capacity of the upstream devices to see if they safely limit the maximum current to a suitable level in case of a low impedance fault.

 

[Simonslimline]

Instead of doubling the reading it should be X 1.732

 

[SirKit Breaker]

Instead of doubling the reading it should be X 1.732

But doubling it, is easier to remember and it err's on the side of caution.

 

[lee0069]

Thanks for all your replys

I did phone elecsa tec supprt and they have said to me

"i know this dont sound right but the BS88 company fuses will stop the fault current before ot has time to do damage"

Still dont sound right to me LOL

 

[SirKit Breaker]

Thanks for all your replys

I did phone elecsa tec supprt and they have said to me

"i know this dont sound right but the BS88 company fuses will stop the fault current before ot has time to do damage"

Still dont sound right to me LOL

Especially as the largest rating PFC of any fuse that i am aware of in normal service is a BS88-3 at 16kA. I am not saying that there isn't any rated any higher, but that is as far as i need to go with what i do. And to be honest, when i worked in the real world i only ever came across a PFC anywhere near 10kA once.

Cheers.............Howard

 

[Engineer54]

Especially as the largest rating PFC of any fuse that i am aware of in normal service is a BS88-3 at 16kA. I am not saying that there isn't any rated any higher, but that is as far as i need to go with what i do. And to be honest, when i worked in the real world i only ever came across a PFC anywhere near 10kA once.

Cheers.............Howard

You are talking about domestic service fuses, this situation doesn't sound to me like a domestic installation, more like an industrial or large commercial type installation!!

If the main switch boards protective devices are BS 88, then they will have a KA rating of at least 33KA and far more likely 80KA.

Whenever we fit a local small load final circuit DB's (lighting and small power) in TX intake rooms and the like, we always provide 25KA MCB's as a well found precaution, against let through currents etc... Depends very much on the size and number of TXs (ours generally being multiple 1600KVA), etc and the upstream protective device characteristics....

 

[Richard Burns]

Just as a possible explanation of the current limiting effect of protective devices.
If the fuse has disconnected completely before the current reaches it maximum level then the current in downstream devices will not rise above the point at which the fuse disconnects.
Because fuses and circuit breakers act quickly and comparatively the fault current rises somewhat slower then for fuses the element will first start to melt and then (pre arcing) when it starts to separate the arc will continue carrying current until the arc is suppressed (up to the total let through energy). This may happen in one thousandth of a second at high currents so the current will not have reached maximum yet.
This example picture may show what happens more clearly.

You can see that at 20kA fault current the maximum peak current experienced downstream if you have this particular BS88-2 fuse would be about 8.8kA.
Admittedly more than your 6kA device can handle but still a lot safer. If for instance there were a 10kA device in place then this would be safe even with a PFC of 20kA because of the current limiting of the upstream fuse.