Ka rating

[Baddegg]

Anyone point me the right direction for the ka rating of bs 88 class Q...

 

[James]

200kA I think.

http://www1.cooperbussmann.com/pdf/ab35d3f3-a641-4185-b35e-ba44d53788b5.pdf

 

[Baddegg]

Holy ----!.....this is covering domestic property....

 

[James]

Holy ----!.....this is covering domestic property....

the fuse will not be the limiting factor, that is the maximum current it can take and guarantee breaking. over that current there could be a sustained arc that continues after the fuse has failed.

the important figure you are looking for is prospective fault current I presume?
this is limited by the loop impedance and can be found by either enquiring with the supplier or by testing.

 

[Baddegg]

Thanks James measured at 3285

 

[James]

wow, that is still quite high for domestic.
I try to keep out of domestic but have rarely seen above 2ka when doing pfc tests at that sort of install.

strangely enough, the times where I have had huge figures are in buldings that have ther own on site transformers and the measured value is just balls.

3 ft from the 1Mw transformer and you get a silly reading like 1.8ka
that is when you have to enquire with transformer people and discover that 20 x that figure is possible.

the theory behind it is a little over my head, but the basic premmise is that the meters we use to measure pfc work on resistance and when you get real close to the transformer you really need a much better piece of kit that can measure/estimate the impedance of the circuit.
like i said a little over my head!!

@Lucien Nunes

I am sure you will be able to provide an in depth reply that puts me to shame!!

 

[Ian1981]

Common domestic primary fuses
Bs1361 type 2 33ka
Bs88 gG 31.5 Ka

 

[James]

we have to be careful not to confuse maximum breaking capacity with prospective fault current.

 

[Ian1981]

Don’t worry I’m not confused

 

[Baddegg]

wow, that is still quite high for domestic.
I try to keep out of domestic but have rarely seen above 2ka when doing pfc tests at that sort of install.

strangely enough, the times where I have had huge figures are in buldings that have ther own on site transformers and the measured value is just balls.

3 ft from the 1Mw transformer and you get a silly reading like 1.8ka
that is when you have to enquire with transformer people and discover that 20 x that figure is possible.

the theory behind it is a little over my head, but the basic premmise is that the meters we use to measure pfc work on resistance and when you get real close to the transformer you really need a much better piece of kit that can measure/estimate the impedance of the circuit.
like i said a little over my head!!

@Lucien Nunes

I am sure you will be able to provide an in depth reply that puts me to shame!!

Taught him all he knows ?

 

[buzzlightyear]

Common domestic primary fuses
Bs1361 type 2 33ka
Bs88 gG 31.5 Ka

page 74 YBB.

 

[Lucien Nunes]

I can't add anything here, as I believe James has already hit the nail on the head, or at least read the same two PDFs on the subject. I too was given to understand that as the inductive reactance of the transformer becomes the dominant component of the impedance (i.e. the pf of the test current falls), so does the accuracy of the measuring circuit. This is in addition to the fact that at low impedances, the measured value tends to approach the instrument's resolution, at which point even an accurate measurement is not readable. You either need a very spendy piece of kit, or sight of the transformer data plate and cable so you can calculate it.

I first discovered the limitation of my then-MFT when we moved into a building with our transformer room located right inside the building. The panelboard was 6 feet away and the general local DBs 6 feet away from that, with the submains just long enough to use standard MCBs.

 

[Baddegg]

And some words...

 

[buzzlightyear]

did you whip that out of the EICR job and you left them with out power ,lol.

 

[James]

Thanks @Lucien Nunes

I first found out the limitations of a mft tester when i was getting some readings far below what I was expecting.

I went from board to board then eventually to the main panel board ant the figures were getting more unintelligible
Eventually I took a reading off the transformer itself and the figures were useless. PFC was getting lower and lower as I got closer to the transformer.

I no longer believe any reading I get if I am within 20m of the transformer.
Even then, I suspect they are not true but you have to believe your test equipment at some times?

 

[Baddegg]

did you whip that out of the EICR job and you left them with out power ,lol.

Give em time to catch the rats...

 

[buzzlightyear]

Give em time to catch the rats

who's the rat catch pied piper ,after a drink .lol.