10 amp mcb protecting Lights

Postby tom tom » Wed Mar 11, 2009 11:04 pm

Hi all,
I wonder if anyone could answer some queries i have regarding my electrics in my house,
Q1- As far as i was aware light circuits could only be protected by 5amp fuse or 6amp mcb, but the electrcian who installed my new mcb cu has put downstairs lights on 6 amp mcb and for upstairs lights he has protected with a 10amp mcb, is this correct to use bigger fuse (10 amp) for lights and if it is then does this mean i could have more lights on this light circuit than the 1100 or 1200watts that are allowed on a 6amp.
Q2- At the risk of sounding stupid is it normal for lights too trip when a light bulb blows, as this has happened twice since he fitted cu in december.
Q3-how do i find out what type of earthing system i have, the main earth is 16mm which is connected too the earth terminal in cu and goes too a terminal outside the cu, next too main 100amp fuse.

Thanks in advance for any answers too my questions
tom tom
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Postby Ken Woodchef » Thu Mar 12, 2009 12:16 am

Hi Tom Tom. I'm no expert but having just read through the guide to 17th edition wiring regs (personal Research), they do make referance to the types of beakers used in domestic lighting. For type B C/B is calls for 10A, for type C it calls for 6A & for Cartridge Fuse it calls for 5A, this is on installations using 1 & 1.5mm2 T&E cable. It may be just the type of brakers which have been used as opposed to design / load considerations.

Modern breakers are quite sensitive to surges that often occur when a light blows. A higher current breaker may allieviate this (no more that 10A) If you had traditional Cartridge fuses before, these are more resilliant to surges, so may just be that? I get it , but would rather it trip than burn my house down:-)

There are 2 main domestic Earths PME (Protective Multiple Earthing) where the earth is provided by the Electricity Board & is bonded to the incoming 'Neutral' & the traditional Earth Rod in the ground. If it's a newish house then you may have a PME. There should be alable indicating a PME, if not, hunt for an Earth Rod.

Hope it helps.

Ken Woodchef
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Postby kbrownie » Thu Mar 12, 2009 5:21 pm

I'll try my best to explain foy you
Q1) The Breaker or Fuse in your CU protects your cable and electrical equipment supplied by that circuit, if your cable size was your standard 1.0mm2 flat grey Twin and Earth for example and was clipped direct. The current that this cable can carry without damage being caused by heat in the cable is 16 amps so a 16 amp breaker is borderline but okay, the idea is that the breaker will trip before the cable melts.
It is a standard that most lighting circuits are wired in 1.0mm cable and protected by 6amp breaker this is the minimum requirement and is done as it is the cheapest method. (I try to install 1.5mm).
As you state it is also standard that all lamps should be rated at 100watts per lampholder and the calculation of 230v x 6amps = 1380watts divide that by 100watts for lamps = 13.8 lamps. But 12 is the normal recomended for this circuit.
But as one of your circuits has a 10amp breaker this would equal 2300watts of power to the circuit divide that 100, would allow 23 lamps. Discharge lamps should be calculated by 1.8 times the watt rating of the tube, so if it was a 75watt tube it would come out at 135watts.

Q2) Yes this is normal it is doing what it is designed to do

Q3) domestic installation have 3 mean types of earth the
TT=earth electrode
TN-S=earth is taken from metal sheath of in coming supply
TN-C-S=earth is taken from the supply neutral at cut-out
You can identify this be inspecting around the cut-out area, where your incoming supply is before the meter you will have a plastic case that the supply fuse is installed
If the mains earth conductor is coming from this enlosure to a mains earth terminal (MET) it is TN-C-S
If the mains earth conductors is taken from the sheath of the cable before the enclosure then to a MET it is a TN-S
If the main earth does not do either of these a goes directly to CU it is a TT system.
Hope I expalined this okay and you found not only rivetting buy useful info.
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