few questions

[andy]

1 how do you work out the PFC?
2 how do you work out the external loop impedance?

EDIT

3 if i was to connect a UPS to power some items (i.e heating, lighting and some sockets) and a generator for when the battery starts going low, should i connect an earth to the neutral side or what?

 

[andy]

anyone?

 

[breezer]

unless you have a very big UPS i think you would be better off with your generator.

 

[andy]

unless you have a very big UPS i think you would be better off with your generator.

the ups will be mainly to support my computer until the generater has time to start up

 

[plugwash]

unless you are buying a monster of a ups then you only wan't to put the ups on your pc and/or other equipment where short term downtime is unacceptable

neutral should be linked to earth at the connection to the generator if it is not linked inside the generator (you can verfy this with a multimeter)

essentially there are two possibilities

1: use a manually started genertor with a manual supply changeover switch
2: use a generator with automatic start and changeover facilities

the first one obviously requires human interacton for the changeover the second one is likely to be much more expensive

also i would strongly reccomend against running heating off a generator otherwise you will start needing a very big generator very fast. gas (natural or bottled) or oil heating seems far more sensible

 

[andy]

also i would strongly reccomend against running heating off a generator otherwise you will start needing a very big generator very fast. gas (natural or bottled) or oil heating seems far more sensible

not sure what you mean by that, but the heating is gas and needs electricity

also, i would have the generater start when the power fails and use a 2NO+2NC contactor as a changeover switch


so back to the original questions,

1 how do you work out the PFC?
2 how do you work out the external loop impedance?

 

[Lectrician]

The PSC and Loop Imp (Ze) can both be measuered on site with the appropriate test kit. This is the best method, as if you use the maximum declared values from the REC, it can be hard to meet disconnection times on longer cable runs.

If you know the loop impedance, you can calculate the PSC using ohms law. HOWEVER, you need to know the loop impedance from L to E and L to N, and use the lowest reading to calculate the highest possible PSC. This is predominantly from L to N.

If you are connected to a 3 phase supply, the highest PSC will be between the phases, many test instruments are unable to test this value, so the highest recorded value from L to E or L to N is doubled, this gives a value on the side of caution.

Max declared values are:

TN-S-C (PME) - 0.35ohm
TN-S - 0.80ohm

They also quote estimated maximum PSC values, obviously, the lower the Ze, the higher the PSC.

Single phase - 16kA
Three phase - 25kA

So if you live furthest from the supply transformer, you can expect a high Ze and low PSC - If you live close to the supply transformer, you can expect a low Ze and a high PSC.

Make sense?

 

[andy]

Make sense?

yes.

you say it can be workewd out on site with appropiate test kit - how do i do this?

 

[Lectrician]

Buy a combined Loop and PSC Tester - approx £450

Read the instructions, and your away..........WORKING LIVE THOUGH