pscc of breakers

[ncoups]

Hi,

I'm part of the DI scheme and don't normally do any sort of commercial install work. However my friend has bought a garage space which he is converting into a recording studio.
He hasnt had the supply connected up yet, and the DNO has requested for a basic installation to connect the supply to.
Do you think they will be expecting 10ka pscc mcb's? I can't get any conformation from them. If I go ahead and just installed 10ka breakers, that doesnt break any regs does it?

Cheers

 

[ban-all-sheds]

Do you think they will be expecting 10ka pscc mcb's?

Why would they?

Is the garage right next to the substation?

If I go ahead and just installed 10ka breakers, that doesnt break any regs does it?

Why should it?

 

[ncoups]

Isn't it normal practice in commercial premises to install minimum 10ka breakers regardless of measured PFC?

I know it shouldn't break any regs, but I dont want to upset the DNO, there has already been a 9month wait for connection through small miss communications.

Thanks

 

[plugwash]

What type of supply are we talking here? Single phase or three phase? what current?

 

[Spark123]

Can you measure the PFC in an adjacent building?

 

[scousespark]

"A man oughtta know his limitations"

Quoting Dirty Harry.

 

[ncoups]

It's a single phase install, and we've had no confirmation of what current is being supplied, or confirmation of anything for that matter.
I might measure in adjacent building, isn't ideal though.
And thanks for that scousespark, but I'm pretty highly qualified, and this is just a fairly unusual situation I thought some helpful individual here may have been in before.

 

[riveralt]

It's a single phase install, and we've had no confirmation of what current is being supplied, or confirmation of anything for that matter.

Current isn't supplied its drawn by the equipment installed.

I might measure in adjacent building, isn't ideal though. And thanks for that scousespark, but I'm pretty highly qualified, and this is just a fairly unusual situation I thought some helpful individual here may have been in before.

I am confused by what you are trying to achieve. You say you are highly qualified but you seem to be leaping to conclusions without having done the basics.
The DNO has requested a basic installation to set up to - in their view this means one that has been tested and certified. But of course you cannot complete all the tests without the power - a brilliant catch22 situation.

In this situation you should be designing the circuits based on the likely demand expected.
Its single phase so put a standard Domestic insulated or metal CU in that has sufficient space for the circuits you have designed. Then connect a short radial circuit to a single double socket from the CU. Run you tails to an dual pole 100Amp isolator, set up the Main Earthing Terminal and call the DNO to connect their cables to the isolator.

Once they have connected up their cables you can quickly conduct the appropriate Ze, prospective earth fault and prospective short circuit current tests which will determine the Ka range for your mcbs. Sometimes the DNO want a certificate to indicate that the system has been tested live you can do this once the Ka range requirements are known.

 

[yorkspark]

If your on the domestic installer scheme why are you getting involved in commercial work?

If your pretty highly qualified why are you asking questions on a DIY forum and it's not a fairly unusual situation, your just out of your depth.

If your qualified why do you think it could be against the regs to fit 10ka mcb's ? Do you actually know what the 10ka means?

 

[kai]

=Do you actually know what the 10ka means?

The 10ka breaker, means it can safely interrupt a short circuit of up to 10ka safely, it does no harm to have higher kA breaking capacity ones - it's just that your wallet will be stretched more when you purchase higher ka ones than required.

Installing 80ka breaking capacity fuses (BS88), does no harm - millions of installations have BS88 fuses in them, yet the max short circuit current at the point of installation is only a few kA at the worst.

 

[ban-all-sheds]

Kai - is ncoups another of your aliases then?

 

[JohnW2]

Once they have connected up their cables you can quickly conduct the appropriate Ze, prospective earth fault and prospective short circuit current tests which will determine the Ka range for your mcbs.

I often wonder about these very high PFCs/PSCCs one sees talked about, and wonder whether they are ever seen with domestic supplies, even if the property is sitting right next to a substation. Even if one ignores the transformer windings, just 25m of 25mm² feed cables (and I think many domestic supplies are smaller than 25mm²) would limit the PFC/PSCC to under 6kA (about 5.25kA at 230V).

I wonder what the highest PFCs/PSCCs (or lowest Ze) people generally see in domestic supplies?

Kind Regards, John.

 

[toasty]

I agree, not to mention that a transformers max output current isn't just based on its output impedance. The primary windings, core and supply impedance have a part to play.

 

[JohnW2]

I agree, not to mention that a transformers max output current isn't just based on its output impedance. The primary windings, core and supply impedance have a part to play.

Indeed. I wonder what the DNO actually say. For most purposes (e.g. standard I&T, EICRs etc.) it is acceptable to determine Ze and PFC of TN systems 'by enquiry'. I know that their standard answer for Ze is 'no more than 0.35Ω/0.8Ω' - but what (if anything!) do they say about the minimum Ze, or the PFC, when asked?

Kind Regards, John

 

[plugwash]

IIRC there is a reg that lets you assume for most single phase supplies that the service fuse is providing back-up protection and so you don't need to worry about the breaking capacities of the MCBs