Measuring inrush currents.

[NDG Elecs]

Hi,

Apart from spending £300 on another clamp meter (although I am tempted!) is there a cheaper way to measure inrush currents? Can't seem to find anywhere to hire one either for less than £100.00. Oscilliscope either. This is a one off job and am curious about the actual inrush on some LEDs. I have analogue and digital multimeters, clamp meters and MFT. But cannot think of anyway to measure it.

Thanks, Neil

 

[Andy78]

I have never measured inrush, but I would assume you would need a clamp meter with peak hold function and a fast sampling speed.

 

[HandySparks]

I have never measured inrush, but I would assume you would need a clamp meter with peak hold function and a fast sampling speed.

I spent some time a while ago measuring inrush on 24V DC LED drivers after a problem with a tripping MCB after I installed some under cupboard lighting. Turned out that the inrush on the ones I fitted first was huge. I have a Metrel MD9270 which seemed pretty good for the job.

 

[davesparks]

In an ideal world you'd want an analogue instrument with a peak hold function

 

[NDG Elecs]

I have never measured inrush, but I would assume you would need a clamp meter with peak hold function and a fast sampling speed.

There is a Fluke clamp meter that measures the first five cycles of the sinewave (or something like that!).. So as you say, that would be the fast sampling/testing part. Anologue meters would probably just get the needle doing a quick dance.

 

[NDG Elecs]

I spent some time a while ago measuring inrush on 24V DC LED drivers after a problem with a tripping MCB after I installed some under cupboard lighting. Turned out that the inrush on the ones I fitted first was huge. I have a Metrel MD9270 which seemed pretty good for the job.

Cheers, was just looking through the spec for that, it doesn't seem to mention that it measures inrush (that's to say the quick initial surge that wants to be recorded).
Out of interest what kind of peak were you getting before it steadied? Numbers please!

 

[NDG Elecs]

In an ideal world you'd want an analogue instrument with a peak hold function

That's what I initially thought, but as the inrush is so quick I figured the needle would swing left to right really rapidly before steadying at the normal current. And consequently I am not sure how accurate the peak hold would be.

Does such a beast exist, if so please tell!

 

[HandySparks]

Cheers, was just looking through the spec for that, it doesn't seem to mention that it measures inrush (that's to say the quick initial surge that wants to be recorded).
Out of interest what kind of peak were you getting before it steadied? Numbers please!

Instruction manual here:
http://www.metrel.si/fileadmin/BAZA..._Function_Multilingual_Ver_1.3_20_751_451.pdf

I had it set to 'peak' with max hold. From memory, I was getting around 150A from three 36W drivers on one circuit. However, this may have been for less than one cycle. Nevertheless, it was a good tool for comparing one driver with another, even if the absolute values weren't necessarily that useful. I think that, momentarily, the current was only limited by the line impedance of the circuit.

 

[NDG Elecs]

Cheers Handy.

That may make a nice back up to my present ELCM. I think I will contact Metrel and see what they say. The spec seems to imply it measures the peak value for 25 micro seconds, which is a very short duration. But you have obviously got results from it. Maybe I have misread something.

The Fluke 374 covers the first five cycles ie 100 milli seconds.

 

[HandySparks]

Cheers Handy.

That may make a nice back up to my present ELCM. I think I will contact Metrel and see what they say. The spec seems to imply it measures the peak value for 25 micro seconds, which is a very short duration. But you have obviously got results from it. Maybe I have misread something.

The Fluke 374 covers the first five cycles ie 100 milli seconds.

My interpretation of the 25 micro second figure is that it samples the waveform (current or voltage) at a rate of 40kHz. I've just had a quick check. The 'peak' function has 'max hold' built in, so it displays the maximum sample measurement (not the RMS figure) since the function was selected.

Measuring the supply to my 5W LED table lamp (not during start up), the RMS current is 55mA (power factor is not 1). The 'peak' value is 300mA, so it'll be a pretty odd waveform.

 

[NDG Elecs]

My interpretation of the 25 micro second figure is that it samples the waveform (current or voltage) at a rate of 40kHz. I've just had a quick check. The 'peak' function has 'max hold' built in, so it displays the maximum sample measurement (not the RMS figure) since the function was selected.

Measuring the supply to my 5W LED table lamp (not during start up), the RMS current is 55mA (power factor is not 1). The 'peak' value is 300mA, so it'll be a pretty odd waveform.

Cheers mate. Must admit some of the terminology is a little over my head! Don't fully understand RMS yet! Must read up on it.

I will defo contact Metrel. If it is the sample rate, rather than the overall duration then that is excellent. Thinking about it I would assume it is, as 25 micro seconds is not very long! The Fluke has a specific inrush button which sets the tester to measure the first five cycles ie 100 milli seconds only I believe. Whereas to measure the peak with 'max hold' (as in the Metrel) would likely keep measuring for a longer period. I would imagine inrushes can peak way after 25 micro seconds has passed!

How long have you had yours... Has it been OK... Would you buy it again??

A mate had a Metrel MFT a few years back and it seemed pretty decent to me.

Thanks again, Neil

 

[HandySparks]

How long have you had yours... Has it been OK... Would you buy it again??

Had it a few years. It mostly gets pulled out to investigate tripping RCDs. Clamp the tails and measure the differential current while turning things on and off.

It's been reliable and gives a stable reading even in a packed board with other current carrying wires nearby.

 

[sam400]

I spent some time a while ago measuring inrush on 24V DC LED drivers after a problem with a tripping MCB after I installed some under cupboard lighting. Turned out that the inrush on the ones I fitted first was huge. I have a Metrel MD9270 which seemed pretty good for the job.

Sorry to hijack - I've just got this same problem with some JCC skyblades tripping the B6 MCB with inrush current.
it says in the MI that they should be put on plug top or FCU ideally but if they are wired to a lighting circuit the MCB 'MAY' need upgrading. Very helpful. I don't think my meter has the resolution to test this so I am going check Zs and put them on a C type.

 

[NDG Elecs]

Chances are the Zs will be OK for upgrade on lights if in a house. I have rarely had Zs on lights creeping up past 3 ohms, and for a C6 60898 it's around 2.9 I think.

Are fireblades downlights? Handy of JCC to suggest putting them on a plug if they are! Could make installation a royal pita!

 

[sam400]

Chances are the Zs will be OK for upgrade on lights if in a house. I have rarely had Zs on lights creeping up past 3 ohms, and for a C6 60898 it's around 2.9 I think.

Are fireblades downlights? Handy of JCC to suggest putting them on a plug if they are! Could make installation a royal pita!

Yea don't think my Zs was too high from memory, just spoke to JCC and they said they have to be on a C type breaker and to allow 2A per driver! I ve got 3 of them so that's a whopping 6A for some 3 X 8w under pelmet lights! Looks like it's going to be a C10 then to allow for other lighting. Bad design on my part but would be nice if they gave you a bit more info!