will my lighting curcuit disconnect in 0.4sec

[ssl0kxp]

Hi.

A very quick question which, I hope has a very quick answer.

I read in my ‘Which? Wiring & Lighting’ that a wall mounted outside light needs be on a (in my case lighting) circuit that should, in the event of a fault disconnect in 0.4seconds. If not then a suitable RCD should be fitted.’

My consumer unit has MCB’s but does not have a RCD, how can I determine if my circuit will trip in that time? I thought the regulations stated that RCD’s should not be fitted to lighting circuits…
Thanks in advance.

(I did search first... )

 

[BJS_Spark]

You need to measure the earth loop impedance (Zs) at that point and cross check in the regs that it is within the stated Zs figure for your mcb type.

 

[Adam_151]

If you know the type of earthing you have, the amount of cable from the CU to the light, and its csa, along with the protective device type and rating then it can be worked out.

You can take the worst case Ze, get R1+R2 from tables, add them to get Zs, and compare this with the table value for 0.4 sec disconnect by the type of preotective device that is installed

 

[ban-all-sheds]

Or measure R1+R2, and add it to worst-case Ze...

 

[Adam_151]

Or measure R1+R2, and add it to worst-case Ze...

He probably isn't anymore likely to own a milli-ohm meter than he is an ELI meter. Can imagine what kind of garbage you'd get if you tried to use a typical multimeter from the sheds to try and get R1+R2...

 

[andy]

Hi.

A very quick question which, I hope has a very quick answer.

I read in my ‘Which? Wiring & Lighting’ that a wall mounted outside light needs be on a (in my case lighting) circuit that should, in the event of a fault disconnect in 0.4seconds. If not then a suitable RCD should be fitted.’

My consumer unit has MCB’s but does not have a RCD, how can I determine if my circuit will trip in that time? I thought the regulations stated that RCD’s should not be fitted to lighting circuits…
Thanks in advance.

(I did search first... )

dont you mean 5 seconds since its fixed wirin?

youll have to measure the earth loop impedance and check it with OSG figures for disconnection times.

 

[Spark123]

Isn't the max EFLI the same for 0.1 - 5s operation of an MCB?

 

[andy]

Isn't the max EFLI the same for 0.1 - 5s operation of an MCB?

for MCBs yes, but not for fuses

 

[ssl0kxp]

Hi guys,

Thanks for the comments, what does EFLI mean? i.e. "Isn't the max EFLI the same for 0.1 - 5s operation of an MCB?"

Andy it was 0.4 seconds not 5.

I was looking for a 'rule of thumb', but atleast I now know what to ask an electician to check for.

Thanks.

 

[Adam_151]

ELFI = Earth loop fault impediance, an electriciain will have a meter to measure this, however if disconnect times check out with the worst case values of ELFI, then they'll be ok with the actuall values.

TNS - 0.7 ohms
TNC-S - 0.35 ohms IIRC

IIRC with mcbs, there is no difference in what the ELFI can to make it trip in 0.4 or 5 seconds, its a magnetic thing, it either trips, or be it doesn't, not like buildin upto tripping like the thermal element, if it trips it'll do it in 0.1 seconds, this is the function that allows it to meet both 04 and 5 sec disconnect times (also resposnible for the breaker tripping when a lamp fails...), with fuses, as its got to burn out a wire and not just activate a magnetic trip, then ELFI for 0.4 and 5 will be different.

It should be noted that mcbs also have a thermal element, this is to deal with smaller overloads over a longer period of time, and can take upto an hour to trip depending on how overloaded it is

 

[Spark123]

Hi guys,

Thanks for the comments, what does EFLI mean? i.e. "Isn't the max EFLI the same for 0.1 - 5s operation of an MCB?"

(EFLI = Earth fault loop impedance.) Supply voltage ÷ EFLI = Fault current.

Andy it was 0.4 seconds not 5.

Disconnection time required is 0.4s if there are exposed conductive parts of the fixed equipment outside the equipotential zone.
The fault current required to trip the MCB in 0.1s is the same as the fault current required to trip it in 5s, providing the circuit was installed correctly i.e. the EFLI is low enough the circuit should be able to disconnect in 0.1s.

 

[ban-all-sheds]

Disconnection time required is 0.4s as there are exposed conductive parts of the fixed equipment outside the equipotential zone.

Nobody has asked him - is it safe to assume that it's a Class I luminaire?

 

[ban-all-sheds]

what does EFLI mean?

http://www.tlc-direct.co.uk/Book/5.3.1.htm
.
.

 

[Spark123]

Fair point, should have been "if there are"

 

[plugwash]

if your lighting cuircuit is protected by a B6 B10 or C6 MCB, not insanely long and doesn't contain bad contacts and your Ze is within allowed specs then you should have no problems meeting 0.4sec disconnect times.