Right i was trying to work out I.n in my house for a downstars lighting circuit and i was looking at haveing 5 100w bulbs 2 in the hall 2 in the kitchen and 1 in the sitting room when i divie 500w by 230v suply i get 2.17A so why cant I.n be a 3 amp circuite breaker why are lighting circutes 6amp?
Help with a study question on fuses
- Thread starter liam2010
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They can if you want.
Why not?
That's not rhetorical - what reasons are there for them not being 6A?
i was always tought that you would put the next nearest fuse size up from the design current in the DB is there any specific reason you woul go one size further than that?
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Historically, lighting circuits were 5A or 1200W and there was often only one circuit per dwelling.This gave enough capacity for a 100W lamp in each room of an average house, with some spare.
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We're back to fuses/circuit breakers being there to protect the cable.
Lighting circuits are normally 1mm² which is rated at 16A in ideal installation conditions BUT, as deduced in a recent post, lighting accessories and the wiring in pendants and fittings is only rated at 6A.
This would not preclude the fitting of a 3A protective device but it would be a disadvantage by being able to connect fewer lamps.
Lighting circuits are normally 1mm² which is rated at 16A in ideal installation conditions BUT, as deduced in a recent post, lighting accessories and the wiring in pendants and fittings is only rated at 6A.
This would not preclude the fitting of a 3A protective device but it would be a disadvantage by being able to connect fewer lamps.
And given the ease with which a failing lamp will trip a B6..... 
Indeed; as you will recall, I'm a sympathiser with the op's view that protective devices should ideally be rated 'as low as possible' for the load in question. However, there is also, of course, the practical issue that 'recommended' MCBs less than 6A do not exist for many/most CUs.
Kind Regards, John.
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For a circuit which can have changing loads we want to fuse to allow for maximum flexibility but still protect the cable and parts like the ceiling rose. There is a point that the BA22d bulb holder is rated at 2A but bulbs should contain a built in fuse so if there is ionisation as they blow it will rupture built in fuse rather than take out whole lighting circuit.
There must be some discrimination between bulbs built in fuse and the protective device fitted. But the ceiling rose acts as a junction box and rated at 6A limits the maximum size to 6A. However a fuse is slower to blow than a MCB will trip so I noted bulbs will take out a B6 when they blow. So one needs to measure the earth loop impedance and if this allows it then a C6 MCB may be a better option to ensure discrimination. I note now bathrooms need RCD protection for lights it is common to fit a C6 RCBO in the lighting circuit.
There seems to be some debate as to the requirements between B, C, and D rated RCBO's as clearly the ELI is no longer a problem. However the Line - Neutral impedance to my mind still should be low enough to ensure the trip will open using magnetic part should there be a short circuit.
The problem with DIY is the lack of meters. We use ohmmeters with < 200ma to test low ohms and 500v to test high ohms, a RCD tester able to measure the milliseconds needed to trip, and a loop impedance/prospective short circuit current meter to ensure the calculations were correct and the trips will trip as required. We should also consider volt drop but that is not really a safety issue in the same way. However most DIY people don't own or hire these meters.
There must be some discrimination between bulbs built in fuse and the protective device fitted. But the ceiling rose acts as a junction box and rated at 6A limits the maximum size to 6A. However a fuse is slower to blow than a MCB will trip so I noted bulbs will take out a B6 when they blow. So one needs to measure the earth loop impedance and if this allows it then a C6 MCB may be a better option to ensure discrimination. I note now bathrooms need RCD protection for lights it is common to fit a C6 RCBO in the lighting circuit.
There seems to be some debate as to the requirements between B, C, and D rated RCBO's as clearly the ELI is no longer a problem. However the Line - Neutral impedance to my mind still should be low enough to ensure the trip will open using magnetic part should there be a short circuit.
The problem with DIY is the lack of meters. We use ohmmeters with < 200ma to test low ohms and 500v to test high ohms, a RCD tester able to measure the milliseconds needed to trip, and a loop impedance/prospective short circuit current meter to ensure the calculations were correct and the trips will trip as required. We should also consider volt drop but that is not really a safety issue in the same way. However most DIY people don't own or hire these meters.
A 100W filiament lamp is a far from perfect resistor. Measure the resistance of a cold lamp and you'll find around 40Ω
The initial transient current, on switching on, could be as high as 6.4A That might cause a problem with a 3A MCB.
The initial transient current, on switching on, could be as high as 6.4A That might cause a problem with a 3A MCB.
That makes sense, but I have to say that I can't recall ever having experienced an MCB tripping at switch-on, even when several lamps (sometimes well over 200W total) are coming on simultaneously, unless a lamp pops at switch-on. In my experience, it's invariably when lamps pop that (6A Type B) MCBs can trip (either poor internal fuses in the lamps or poor discrimination, I guess).
Kind Regards, John.
The RCD, or RCD part of RCBO, should only be used as additional protection or protection if the appropriate Zs for the protective device cannot be met.
Merely recording that the Zs for an RCD is below 1666Ω (N.B. not 1667) is not acceptable.
If the earth impedance is as it should be then the neutral will not be a problem or vice versa.
I think that the resistance of a cold lamp is lower than that, but the inrush current is so very brief it will not trip the magnetic part of any circuit breakers
I shall try to convince you again.
That's the whole point - the cpd is not there to be 'ideal for the load'.
It does nothing for the load. It is to stop the cable melting.
It may be difficult to convince you with single load circuits as these cannot be over-loaded but may suffer a short-circuit. However, if the cpd disconnects before the cable is damaged it has done its job. A lower rated cpd could do it fractionally quicker (an extremely small fraction) but as there was no damage it wouldn't matter.
With multi-load circuits the above still applies as far as short-circuits are concerned. These circuits may be subject to over-load but if the cpd disconnects the power before any damage to the cable occurs all is well.
A lower rated cpd would merely reduce the number of appliances that may safely be used.
A 3A mcb would not 'save' any of the OP's lamps.
If he had only one 60W lamp in his house there would be absolutely no benefit from fitting a ¼A mcb.
OK, but I really doubt that there is anything to say that we haven't all said already, in previous discussions.
We have all agreed that it should be able to stop the supply cable melting - which, indeed, is what is required by the regs. However, that does not, to my mind, alter the fact that it makes sense to make use of the ability of the device to give some degree of protection to other things in the circuit if/when that is possible.
I agree that it cannot save lamps, but that doesn't necessarily mean that it cannot protect anything.
In any event, we've been through all this before and agreed that the conventional 6A protection of a lighting circuit is probably there to protect accessories and pendant cables, even though, particularly if wired in 1.5mm² cable, a much higher rating could be used to protect the supply cable alone.
We've also previously discussed the situation in which internal wires and components in appliances could 'burst into flames' under fault conditions, with currents far lower than those which would damage the supply cable. If the situation is such (e.g. dedicated circuit) that it would be possible to have protection far lower than actually required for the supply cable, that fire risk would be reduced.
What about the fuses in plugs? Even if the flex was capable of standing much higher currents, I would still consider it appropriate to use 3A or 1A fuses where appropriate to the load - again, to afford some protection (and, potentially, reduced fire risk) to whatever is on the end of the cable.
Kind Regards, John.
I have seen a large office where there was a single wall switch for several lamps but the lamps were actually switched by contactors to stagger the switch on as the inrush current (when they all switched on together) was more than enough to trip the breaker.
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