HOW TO READ AC CURRENT WITH DIGITAL MULTIMETER

andrew.

Hello,
I have a true RMS digital multimeter and I need to measure current of different appliances. I am not sure I am connecting the test leads properly.
This is how I connected the DMM:

The live 220V wire goes directly to the appliance.
I interrupted the circuit breaking the neutral between the appliance and the mains, and I connected the two test leads, so one is connected to the cable going towards the mains and the other test lead is connected to the cable coming from the appliance.

[220V AC] _______live wire_____[appliance]
|___________________________|
| neutral wire_________________|
|__________[multimeter]_______|
black lead …………………. red lead

The matter is that the circuit is interrupted, so the appliance does not even switch on.
When I measure the frequency, for example, current goes through the meter, the appliance switches on and I get a reading. But when I want to measure the current everything goes dead. :mad:

It came to my mind that maybe the internal fuse of the multimeter is gone, but before replacing it I would like to know if I am making a mistake in the way I did the connections.m Or what is the correct way to measure current of an ac appliance with a multimeter.
Thank you very much!

2011

The general rule is that volt meters are connected in parallel with the load and ammeters are connected in series (either on the live or as you have done on the neutral).

However when measuring mains current a grip on ammeter such as the one shown are normally used:



They are "clamped" around either the live or neutral cable and provide current measurement of AC loads.

Normally multimeters that measure current are designed for measuring small DC loads, unless they have the clamp on part as shown. I think you will find that this is the case with your meter.

andrew.

thanks for reply.
My multimeter is design to measure AC currents up to 20 amperes.
Can you confirm if the connection is correct? In which case is likely that I need simply to replace the fuse?
Thanks

andrew.

Specifications: Basic functions:RangeBest Accuracy DC voltage:60mV/600mV ±0.5% °6V/60V/600V/1000V ±0.8% AC voltage:60mV/600mV ±1.0%
6V/60V/600V/750V±1.2% DC current:600µA/6000µA, 60mA/600mA ±1.2%
6A/20A ±1.5% AC current:600µA/6000µA, 60mA/600mA ±1.5%
6A/20A ±2.0%

crosstownk

As stated earlier - current needs to be tested in series which means that the ammeter will have to be in the circuit. From DIY perspective the 'Amp-Clamp' is the best option. Your meter is rated a 6A. Some domestic circuits may be in excess of 6A.

Cheeble

Andrew,

The connection you have drawn will basically measure the current, but be careful as there could be hazardous voltages on the multimeter, even when it's connected in the neutral wire.

Have you connected the leads between the COM and A sockets on the multimeter?

Cheeble-eers

andrew.

Thank you all!

So, at least now I know that the connections are right.
So, by logic, if the appliance does not even switch on it means that current interrupted somewhere, and most likely that's the fuse of the multimeter gone. The fuse is a 20A.

can domestic appliances exceed 20A?

Andrew

PS this site is great! thanks a lot!

andrew.

Cheeble,
regarding the leads, the A lead is connected to the wire coming from the appliance, the COM is connected to the wire going towards the mains.
Is that right?

Andrew

Cheeble

The way you describe the leads is correct.

Assuming the appliance is working before you connect the multimeter, then it's possible that the fuse is blown (or that the multimeter is otherwise faulty - try replacing the fuse and then measuring a small current with it before you reconnect to the appliance).

Whilst the rated current of any appliance run from a 13A plug will clearly not be 20A, it is possible for appliances to have a surge current on start up which exceeds this. A brief current spike like this would not trip the main circuit breaker, but could easily blow the multimeter fuse.

Once again, be very careful of the hazardous voltages (rubber gloves and a mat would be a good idea). The previous posters are right in suggesting a clamp-on meter if you can borrow one from somewhere: they're much safer, albeit less accurate for you application.

Cheeble-eers

Randyleprechaun

On most multimeters you have to move one of the test leads to a different terminal to measure current. The COM one stays in place and the other moves to the A terminal.

andrew.

I replaced the fuses and I can measure current correctly.
Cheeble you were right about the current spike: it blew the fuses.
Thank you all!
Andrew.