Length of Supply

[doni]

I am trying to gain a measure of the Prospective Fault Currents of my supply for building regs. I could ask the supply company, but they will probably overestimate. I have a table for England showing the what these currents would be for 25mm and 35mm supply, depending upon the "length of supply". It doesn't seem to specify what it means by length of supply.

Can anyone help please?

 

[1john]

Without taking a reading with test equipment the DNO will just tell you the max permitted value as per BS7671 i'm affraid.

 

[1john]

Bargain Loop tester!!!!!

 

[doni]

Hi John - thanks.

I am creating a new bathroom by halving the size of the existing kitchen and then releasing the existing bathroom to use as a bedroom. In essence, it means relocating light and sockets - a fairly simple task - and so I have informed BC.

They have sent me a list of things to give to them - inclduing the PSC. The current table I have of PSC (17th Edition) shows that up to 5m with 35mm, the PSC is 12 kA. Now if it was 10m, it would be 9.3 kA. If it's under 10kA from the tables, I'll just make sure the MCB's are rated at 10kA which will do for their needs.

I could buy a meter and measure the supply impedance, but I think this is asking a bit much as I won't really be changing much cable (except the bit bitten by the rat).

So anyone know what is meant by length of supply?

 

[flameport]

Distance would be the length of cables between your property and the transformer/substation or whatever. Further away = lower fault current.

However all consumer units are rated to 16kA, even if the MCBs contained within have lower breaking capacities. The chances of finding a domestic supply with anything near to 16kA are tiny, and there are plenty of properties where the incoming cable is smaller than 25mm.

Can't really see why they are asking for this information, since it won't change anything, and even if they deemed the results to be wrong somehow, there is nothing you can do about it.

 

[doni]

Hi Flameport - thanks.

I am guessing that they are asking to check how competant I am with doing the works. I suppose the set of questions they send out will put some people off. Well the trasnformer has got to be more than 10m away - cos I don;t know where it is, so I'm inclined to answer this question with < 10kA, so that won't affect anything I am doing, and it gives them an amswer to their question. They have also asked for Voltage Drop (among other things) - now I am happy to answer this question, but again, it's a bit daft, as I am only relocating existing circuits - I might asnwer with with a <3% for lighting and <5% for other circuits, or if I can be bothered, I might work it out.

 

[Taylortwocities]

To be Frank (but that's not my name)

You will need to fill in your Installation Certificate.
The first bit of measurement info you have to give is the Ze at origin.
When you measure that, most test meters will also give you the PSC, if it doesnt then you can calc it using the supply voltage (from your test meter) and the Ze value.

I guess you do have a calibrated tester that is capable of carrying out all of the tests that you'll need to do?

Continuity
Insulation Resistance
EFLI
RCD tests
etc
etc

 

[AdrianUK]

"The 'length of supply" is the length of the service cable from the street main distributor to the service cutout. This can be estimated as the shortest distance from the edge of the footpath nearest the installation to the cutout"

Quoted from "Notes of Guidance for The Elctrical Contractor on Earthing & The Characteristics of Supply", East Midlands Electricity Board Feb 1995

Adrian

 

[doni]

Taylortwocities - I am unsure of what is required from BC because I have not done this before, and do not know what they require. If I am required to complete an installation certificate then I will need to either hire, borrow or buy the correct tester. If they will allow me to get a sparky in to do the tests, then that may be an option. However, I am very keen to do this myself, as you can see from the way I have approached this, so would probably get myself a tester.

Moving on to the tester, would this one do all the things they may require of me, or is it lacking something?

http://www.pat-services.co.uk/downloads/manuals/312_Kewtech_KT61_Manual.pdf

Can I ask what the cost of calibration might be please and the required frequency of calibration?

Thank you.

 

[flameport]

That device apparently does all of the required tests.
Calibration is once a year, typical cost £75.

For one job, it will be far cheaper to hire, examples:
http://nihs.co.uk/hire/fluke1653
http://nihs.co.uk/hire/meggermft1552
which even including delivery and collection is still under £60 for a weekend.

There is also the issue of knowing how to use the equipment, and whether the test results produced are acceptable or not, and if not, what the problem is and how to rectify it.

 

[Taylortwocities]

That test would do you fine.

If you look on the PAT Testing web site they give calibration costs. A new instrument will come with a cal cert.

 

[Taylortwocities]

There is also the issue of knowing how to use the equipment, and whether the test results produced are acceptable or not, and if not, what the problem is and how to rectify it.

That is a key point. There's no point doing testing if you don't know what the results mean!
For a one-off, its better to get a man in..