Customers sometimes want both the highest peak current and an rms over a 
defined number of line cycles. If the request is exclusively standards based, 
John's suggestion works well. You'll need to perform five to ten tests randomly 
closing on the supply cycle and pick the highest values.

If an rms value is also needed, you can capture the inrush on a scope and gate 
the measurement over the relevant number of line cycles.


Peter Tarver

From: John Woodgate []
Sent: Thursday, August 10, 2017 05:32

Well, no-one has challenged Annex B since it was first included in the 
standard. Note that the standard applies up to 16 A/phase, so no big motors, 

From: Nyffenegger, Dave []
Sent: 10 August 2017 13:15

I think it also depends on what your EUT is.   Machinery will typically be full 
of all sorts of loads, motors, transformers, power supplies, computers, 
inductive, capacitive, etc.   The inrush would still be the peak current when 
the mains is switched on or also when the machine is started as they are 
typically two separate events.  These could last several seconds depending on 
the machinery and the incoming power sine wave would probably have little 
effect on it.


From: John Woodgate []
Sent: Thursday, August 10, 2017 8:00 AM

Look at Annex B of IEC/EN 61000-3-3. I did a lot of work on this for that Annex 
and you will often get different results each time, because of differences in 
how the current is interrupted at the previous switch-off. You do not select a 
point on the voltage waveform for the switching instant; you can't, anyway, 
because you must use the product's own mains switch (unless it doesn't have 
one). You switch at random points, because that is what happens in practice.

For duration,  you leave the mains voltage applied until the inrush transient 
is over (look at the current waveform); this is usually after three or fewer 
cycles, but for some products it can be rather longer. Normally, the first 
current peak is the highest, but occasionally the second peak is higher.

From: Kim Boll Jensen []
Sent: 10 August 2017 12:30


We have several times been asked to test Inrush Current and have this function 
on our Harmonic tester, but it is not defined how it measure and we get very 
different measurements each time we switch ON the same EUT.

I can't find an IEC definition on the measurement other than "peak current".

I asume that it is most correctly to measure the current  by switching ON at 
the top of the sine (90 deg), but what about duration?

A peak current with a duration of 0.1 ms is not as interresting as the same 
current for 1 ms. And what if there are several current peaks after each other 
such as ringing wave form?

Best regards,

Kim Boll Jensen

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