electrical misunderstandings

[Murflynn]

the new thread about technical queries reminded me of just how incompetent some (apparently qualified) people can be.

 

when I was working in Nigeria in the 80's I shared a bachelor house with 10 engineers and technicians.

as anyone with experience of the region will know, brown-outs and black-outs were common.

brown-outs were the result of reduced voltage.

most black-outs affected only one phase, and all premises had 3 phase incomers, but usually actually utilising only one phase.

amazingly, most houses had the the 3 phase incomers mounted on an exposed board in the porch, so that the incoming cables could be disconnected and reconnected to allow the circuits to benefit from the good phase/s.

 

we had a 'skilled electrical technician' (a Brit) living in.

whenever we had a brown-out resulting from low voltage he insisted that we turn all the lights off.

he explained this by reminding us that if a bulb was (for example) marked as 100W, if we allowed it to draw current from a reduced voltage then the current would increase to compensate, until there was a danger of fire.

he 'proved' his theory because in the relationship W = V x I , the Power (W) was fixed.

 

 

a little education ..............................................

 

 

[pophops]

Doh!

[John V]

Oddly, the resistance of a incandescent lamp changes dependant on the temperature of the filament.

If you measure the resistance of a lamp with a test meter, calculate the current drawn at the rated voltage and from that work out the wattage, you will find that the figure you get is very different from the rating of the lamp.

[DHutch]

Love it.

[jake_crew]

Oddly, the resistance of a incandescent lamp changes dependant on the temperature of the filament.

If you measure the resistance of a lamp with a test meter, calculate the current drawn at the rated voltage and from that work out the wattage, you will find that the figure you get is very different from the rating of the lamp.

 

For this reason, an old-skool 100w bulb makes an excellent current limiter when testing mains powered equipment with a fault or two.

[Murflynn]

Oddly, the resistance of a incandescent lamp changes dependant on the temperature of the filament.

If you measure the resistance of a lamp with a test meter, calculate the current drawn at the rated voltage and from that work out the wattage, you will find that the figure you get is very different from the rating of the lamp.

it's not odd really - the resistivity of most materials changes with temperature.

[Tony Brooks]

it's not odd really - the resistivity of most materials changes with temperature.

 

I thought it was just mild sarcasm pointed at what looked like an incorrect conclusion.

[John V]

 

I thought it was just mild sarcasm pointed at what looked like an incorrect conclusion.

 

yup!