Testing current

[kml_58]

Hi there, could anyone give me some advice as regards testing my immersion heater in the hot water tank i have.

Thought i understood my multimeter however when trying to test the current it caused my fuse to trip. Is it not a case of turning it on the correct setting i.e black lead to common, red to 20A, set the dial to A . Then applying leads to L-N ?

There was a bit of a spark and now a little black mark

thanks

 

[sparkyspike]

Ooops! No that's not how you do it. You need to test for current in series which requires dismantling the circuit, which is dangerous. Best way is to use a clamp meter. (You test between L + N for Voltage not Current.) Or you can (with the power OFF) test continuity in Ohms between L+N and then divide that into 240 to give the expected current.

 

[breezer]

probably need a new meter too

 

[kml_58]

Thanks spike for the reply. Ive tested voltage and appreciate your way of testing the ideal current however is there anyway of proving the element is drawing the expected current.

I have a newish DMM. what would be the correct procedure for testing the element in series.

Thanks again.

 

[breezer]

as has been said, you need to have the meter in series, which is dangerous becuse it has to be live.

ands as i also said you probly need a new meter, since you have probly blown it up, yes you may still be able to read voltage, but not current, take your meter apart, and see if it has an internal fuse


since checking current is dangerous (as you have found out the hard way) may i suggest you get a battery (12 v car one or one form an alarm panel) and put a 12v light on it (halogen lamp as it draws a lot of current) and try with this first.

the current will still be there, but not the 230v mains, and yes a car battery will still fry a meter, if set wrong and it can also melt a cable if you get it wrong

why do you need to know anyway?

also as was said you would do better off checking the element's resisatnce to work out current draw

 

[kml_58]

So one lead clamped to live wire the other to live terminal on immersion? this should give a current reading?

fair point about needing to know i just wanted to know what i'd done wrong really.

will also look at buying a new meter

 

[JohnD]

Your DMM will not be capable of carrying the current that the circuit draws.

It is possible to construct a test circuit with shunt resistors, but that's too much trouble. Measuring the resistance would be fairly accurate (it will change a bit when hot).

What causes you to question the current draw?

If you have fried your meter, you can consider buying a clamp meter next time, but if you are not an electrician you will almost never use it, and they are not cheap. You put the clamp round one conductor only (i.e. the red or brown one only, not red and black, or brown and blue at the same time). The clamp is like a ring that goes round the conductor, it does not make metal-to-metal contact.

 

[kml_58]

Hi john, its not working, but isnt blowing fuses like it usually does.
Will get it changed anyway.

It does read 20A on the meter is it more than this?

was i correct with the method of testing in series.

 

[JohnD]

You do test Amps in series, yes. Thinking of the thickness of the wires to the test probes, I would be surprised if it could carry 20A for more than a fraction of a second without bursting into flames.

An immersion heater should draw about 12A

If you say the immersion heater is not working, test for volatge at the cable, and then see if the thermostat or the element has gone open circuit. You can test with power off, using the continuity test and the meter's own battery.

 

[DESL]

So one lead clamped to live wire the other to live terminal on immersion? this should give a current reading?

Yes but not advisable as previous posters said, to do so will require exposed connections at 240 Volts. As you have already tried to measure current incorrectly, I suggest (as others have stated)you switch power off and measure resistance between L and N on immersion heater and then calculate current using Ohm's Law

V=IR or I=V/R

Hopefully your meter will have internal protection such as a fuse to prevent blowing it up when misused.

 

[JohnD]

just to add...

testing on a live circuit is fairly high risk. Doing it where you are surrounded by earthed copper pipes is particularly undesirable as you may be touching them with some other part of your body when you touch a live conductor.

 

[kml_58]

Many thanks john, desl for replies.

Will have a go at repairing the MM although i dont think ill try that again. I understand how dangerous electics can be.

Will get the immersion changed.

But thanks for clearing up my original question.

regards.

p.sThe resistance was about 15 ohms so about right i suppose.

 

[Softus]

Will get the immersion changed.

Pointless if its the thermostat that's faulty.

p.sThe resistance was about 15 ohms so about right i suppose.

If you suppose that then why will you be changing the element?

 

[breezer]

I never said you meter is faulty, i said it is probably faulty, i also suggested that you

"since checking current is dangerous (as you have found out the hard way) may i suggest you get a battery (12 v car one or one form an alarm panel) and put a 12v light on it (halogen lamp as it draws a lot of current) and try with this first. "

I am sorryb ut you do not have the basic grasp of electricity, why dont you at least read the manual for your meter more (and dont tell me you havent got one)

I base this in the fact the that;
A) you connected your meter wrong to measure current

B) you asked could the immersion heater be drawing 20A although possible in a fault condition for several seconds, how could it draw 20 A when it shoud be supplied by a 13A fuse in the FCU

 

[lookinn]

What you are trying to do is very dangerous. DONT DO IT. In any case most mutimeters won't read high AC current. As other people have said use ohms law.