Last summer I installed a set of PIR controlled outdoor halogen lights on
driveway and inside a group of garages. Idea is that you are lit up as you
drive down to garages and park in them with no switching required.
Wired all lights up and tested the system using 13 amp plug with 13 amp fuse
and extension lead. All worked fine over a period of some months so have
now laid on permanent supply to garages terminating in Screwfix Garage
Consumer unit. This has 40 amp 30 mA RCD and two MCBs, one 16 amp and one 6
amp.Have wired up power point to 16 amp MCB but this circuit is very rarely
used. Lights are connected to 6 amp MCB and used daily.
As you drive towards and inside the garages you activate a series of lights
6x150 and 1x500 watts so the total load is 1400 watts.I make that about 6
amps at 230 volts and usually all works fine but about once a week as the
second or third light in the series comes on the 6 amp MCB trips and you are
in darkness. The RCD never trips it is always the 6 amp MCB.
Why does this trip on a random basis and not every day ?
Do PIR Halogen lights draw a heavier current as they start up ?
Would my best bet be to upgrade the 6 amp MCB to a 10 amp version ?
Many thanks in advance for any helpful answers.
It sounds like you are probably just enough over the limit at times to
trip it. Any lamp, when first switched on cold, will draw a little more
than its rated current.
You can either increase the MCB rating assuming the cable is suitably
rated, or split the lighting load onto two MCB's.
Much more, actually: cold resistance can be as much as 10 times lower
than hot. So indeed, with the trip already supplying say 4A for the
first two lamps, the third one coming onstream could be just enough to
trip the 6A MCB during the, oh, one-third-of-a-second-or-so the lamp
takes to come up to final resistance (of course while it's warming up
its resistance is increasing so the current is going down to the steady-state
value all the time).
A third alternative, which might do the job quite nicely, would be
replacing the 'B'-characteristic MCB which he probably has with a
'C'-characteristic one. This will allow a higher switch-on surge than
the B. An electrickle trade-supply place will have these off the shelf.
For full safety, the earth loop impedance for the size of wiring (1.0mmsq?
1.5mmsq?) should be checked; reference to the OnSiteGuide's Table 7.1
suggests you're OK for a circuit length up to 38m in 1.0mmsq and tor
59m in 1.5.
Better than any of the above, though, would be to replace the 500W
bulbs under PIR control with 300W ones (same length as 500W, so no
need for new fittings). Little more than half the energy consumption,
a bit less light scattered into the once-dark skies, no PIR popping,
and naff-all effect on how well-lit the scene is by comparison to the
pitch black. Further down this line of reasoning is to put up, say, 8
or 10 bulkhead fittings, fitted with say 3 60W incandescents for
instant-on lighting and the rest fitted with say 15W compact flourescents.
The larger number of lower-intensity sources gives more even lighting
without the sharp shadows you get from a small number of spots, and
makes bulb replacement less urgent (you can wait till the morning if
what you've lost is just 10% of your light). That's the approach I've
gone for in lighting up the areas around our house, and it's much less
jarring than the "Colditz Effect" the previous occupants' single PIR spot
HTH - Stefek
That sounds more like runway lighting than a driveway. Don't you have
headlights? Your car headlights are 120W total and show enough to travel
safely at 70 mph. Why do you need 1.4kW when you are parking?
Problem is that headlights are only fitted to front of our cars both of
which need to be manoeuvred a few times in a restricted space before being
reversed into garages to ensure safe exit on to road next morning. In
addition the garages are made of a vicious pebble dashed reinforced
concrete which we do not wish to scrape as we reverse in. Hence need for
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