Log in to the online community

Want to post a reply? You'll need to log in
Fault Current Sufficient to Operate a Protective Device.
Zoomup
2277 Posts
Question
I was undertaking some work today in a chalet park. I was testing R.C.D.s. These are supplied in each holiday chalet at the chalet meter position before any consumer unit. The chalet owners are responsible for any internal electrics after the park's R.C.D.

I wondered why R.C.D.s were first installed by the park owners. After all an R.C.D. or R.C.B.O.s could be incorporated in the consumer units. The supplies to the chalets is TT. O.K. so we do need an R.C.D.

But why TT the permanent wooden chalets? This is not a caravan or camp site at all. And the chalets are not transportable. They are not residential park homes according to the B.S. 7671 definition.

Then it dawned on me. At the last chalet, the remotest from the main intake room the P.S.C.C. is only 400 Amps. It is protected by a B50 type M.C.B. So, under fault conditions the fault may take a long time to clear. Evidently a TT system is safer.

So, what effect would a heavily loaded distribution cable have on the operation of the B50 M.C.B. if the park was fully occupied and the supply Voltage dropped? Could it ever become dangerous and prevent a B50 M.C.B. from not opening, or opening too slowly,  at the remotest chalet say due to a short from L to N?

Z.





 
6 Replies
AJJewsbury
2333 Posts
A B-type should trip immediately at 5x In - so 250A for a B50 - which hopefully you should have at the far end (otherwise you're likely to have v.d. issues for normal working) - unless you've got a significantly reduced c.p.c. (SWA perhaps).

Loads shouldn't make a huge difference - given the (assumed) negligible impedance of the fault itself almost all of the available current will be drawn through the fault at the expense of the normal loads.

   - Andy.
broadgage
517 Posts
I see little if any danger.
If the cable is into the chalet is rated for at least 50 amps, and is protected by a 50 amp MCB then the cable should be safe against thermal damage.
The cable does not "know" the difference between a short circuit current of say 300 amps on a high impedance supply, and a gross overload to 300 amps on a low impedance supply. In either case, the MCB should protect the cable against thermal damage.

Earth faults are more dangerous since a person could be exposed to about half line volts for a dangerous time whilst waiting for the MCB to trip. In this case that is handled by an RCD.

I would however consider this installation to be almost certainly unsatisfactory as regards voltage drop.
A short circuit current of 400 amps implies a 10% voltage drop at a load current of 40 amps, or 12.5% at the full load of 50 amps.
Volts from DNO could well be 220 volts actual.
Voltage at intake to chalet could be about 195 volts.
Voltage at point of use could be below 190 volts, almost certainly too low.
Zoomup
2277 Posts
broadgage:
I see little if any danger.
If the cable is into the chalet is rated for at least 50 amps, and is protected by a 50 amp MCB then the cable should be safe against thermal damage.
The cable does not "know" the difference between a short circuit current of say 300 amps on a high impedance supply, and a gross overload to 300 amps on a low impedance supply. In either case, the MCB should protect the cable against thermal damage.

Earth faults are more dangerous since a person could be exposed to about half line volts for a dangerous time whilst waiting for the MCB to trip. In this case that is handled by an RCD.

I would however consider this installation to be almost certainly unsatisfactory as regards voltage drop.
A short circuit current of 400 amps implies a 10% voltage drop at a load current of 40 amps, or 12.5% at the full load of 50 amps.
Volts from DNO could well be 220 volts actual.
Voltage at intake to chalet could be about 195 volts.
Voltage at point of use could be below 190 volts, almost certainly too low.

And this measurement was taken today with the chalet park closed so no loads were connected by other chalet users. I was concerned about the park when it is fully occupied.

Z.

mapj1
3144 Posts
Quite right, PSSC of 400A, assume supply voltage at origin of 240- effective Zs willl be no better than 240V/400A or about 0.6 ohms.
Now there is no time limit for disconnecting L-N faults, so those are OK, but to fire a B50 smartly on an earth fault could take as much as 250A, and if it is a C50 then 500A.
An RCD or RCBO removes these concerns, totally.
I'm with Broadgage, folk will notice the lights dim when the neighbours put the kettle on. If it is a real problem is less clear, appliances vary in how well they behave on low voltage.
It may be the transformer is private to the site and the taps are set a bit high to allow for this.
M.
lyledunn
512 Posts
I wonder which one of these caravan/chalet parks will be the first to display a sign at the entrance “EV VERBOTEN”!
Zoomup
2277 Posts
mapj1:
Quite right, PSSC of 400A, assume supply voltage at origin of 240- effective Zs willl be no better than 240V/400A or about 0.6 ohms.
Now there is no time limit for disconnecting L-N faults, so those are OK, but to fire a B50 smartly on an earth fault could take as much as 250A, and if it is a C50 then 500A.
An RCD or RCBO removes these concerns, totally.
I'm with Broadgage, folk will notice the lights dim when the neighbours put the kettle on. If it is a real problem is less clear, appliances vary in how well they behave on low voltage.
It may be the transformer is private to the site and the taps are set a bit high to allow for this.
M.

Hello Mike,
                        there us no site transformer. The supply comes in from overheads. There are five overhead lines on poles. I will take a closer look when I get time. The main earthing terminal is connected by a black 25mm single cable coming up from the ground next to the main underground incoming main supply cable.  I do not know its origin.

My next investigation will be of the 25mm2 twin underground S.W.A. cable supply to the most remote chalet. It comes from a distant distribution board and is protected by a C63 M.C.B. This is the cable with the P.S.C.C. of 400 Amps at its end. What will an earth fault produce if somebody drives a metal pole through it?

Note. E.F.L.I. at main intake room = sub 1.0 Ohm. P.S.C.C. = 1.68kA.

Z.



 

Share:

Log in

Want to post a reply? You'll need to log in