110v lamp mystery

crabapple · Sep 27, 2009

was called out to look at a job - a pir floodlight was not working.

apparently the man just changed the lamp an now its tripping the cb. true it was.
there was jb with the feed off a local light, load (pir flood) and a switch.

i checked everything and could find nothing wrong.

the jb without the load worked , but as soon as i connected the load it tripped. i checked the continuity of the load cable and it was good.

i took the lamp out and connected the load and that seemed to work now!!

i did a continuity on the neutral holder to the neutral terminal and it was ok - no shorts
the line was also ok!

i did a continuity on the lamp and it was good, but it was then that i noticed the lamp was 500w @ 110v

now a 500w @230 = 2 odd amps
would i be correct in saying that 500w @ 110v = 4 odd amps and thus the overload????

The Truth · Sep 27, 2009

actually its more like 9.5 amps!

your 500w lamp draws 4.5 amps @ 110v and 9.5amps @ 230v

the lamp is basically just a resistor...and for a set resistance..if you double the voltage across it...you double the current going through it.

( using P = I x V and V = I x R ) you can work out that the resistance of the incorrect bulb is about 24 ohms.

current = 230v divided by 24 ohms = 9.58 amps

thats why your 6amp mcb is blowing!

crabapple · Sep 27, 2009

so im definately wrong in saying the following??

I=P/V

500/230 = 2.17 A

P=V^2/R so R=V^2/P

230 * 230 /500 = 52900/500 = 105.8 ohms

500w/110v = 4.54a

R=V^2/P

110 * 110 /500 = 12100/500 = 24.2 ohms

The Truth · Sep 27, 2009

your calcs are correct but remember that your putting a 110v bulb in a 230v light......

the 110v rated bulb you worked out has a reisistance of 24.2 ohms ( much lower than the 230v rated bulb which was 105 ohms )

if you use the 110v bulb in the 230v floodlight.....you get ( 230v / 24.2 = 9.58 amps ) ( I = V/R )