Inrush Current

Measuring inrush current is not difficult to do, but you have to be aware of all the issues. Here is a source discussing the topic of measuring and controlling the transients due to turning equipment on and off: http://trs.nis.nasa.gov/archive/00000466/01/cr209574.pdf. I would add to that source that for 50/60 Hz ac-powered loads, I would set the oscilloscope to not much faster than 1 millisecond per division. You can measure some really large inrush current magnitudes during the first 10 microseconds of inrush, but it isn't really important. What you are looking for is not loading the UPS output excessively, and for that to happen you need the inrush to last an appreciable fraction of a millisecond. Also you need to take into account how different loads behave. My PC generates a small inrush event on my UPS at turn on, but doesn't go into battery overload. Once it's on, even cycling from sleep back to active doesn't trouble the UPS. If you were to measure the max initial inrush current of a PC power supply, you would find that it can easily draw ten times steady-state current draw. But the switched mode power supply input filter caps (several hundred microfarads) which are responsible for the inrush current surge charge up in a few microseconds and are not a real problem. My 20" CRT monitor sends the UPS into battery overload when it's first turned on, but it's on 24/7, so it's not an issue. But my laser printer, even though it's on all the time, will cause battery overload every time it's asked to print. So you have to look at how equipment operates when assessing the severity of inrush events.

Just use an Oscilloscoop and trigger on the main switch power on event.

Use a voltage probe and curent probe(clamp) to measure the inrush current.

And better to use a storage CRO.

Or

peak inrush current is almost a inverse function of total line residence only, as inductance and capacitance dose not come in picture by that time.So one can measure line Resistance with a micro-ohm meter and calculate peak inrush current.

Depends on the device off course some devices input resistance change during operation.

If you install an uninterruptable power supply for the case of mains failure, you will very likely not have to deal with inrush currents and the like, because the UPS will simply take over the current and the operational state of your equipment will basically not change. If you have another set-up then the best bet might be to measure the current consumption during the start-up procedure and compare with the specs plate. I think that specs plates give usually the nominal consumption and say nothing about inrush and the like. However, not necessarily each device does consume more power just after switching on. I did similar tests on various devices and had some few which begun to consume a few tens of Watts more after the processors had completed the boot sequence.

As per the standards being followed, I feel that for Inductive loads like motors , transformers .etc. the inrush current can be taken as 6 times the nominal rated current. As for other equipments like PC / electronic loads with ac inputs , the inrush current can be taken as 3 times the rated nominal current.

I feel that the above thumbrule would solve your problem.

Binny T R

Scientist/ Engineer - VSSC/ ISRO , Kerala, India

If you can get your hands on a Dranetz 4400 it will probably tell you what you need to know. I used one several years ago and it is quite an impressive machine. Easy to use and quite good for inrush, power quality, and a bunch of other things.

Inrush current can only be measure perfectly when the whole unit(loads) are shut down for about two to three hour from my own practical observations because i have done such measurement with a switch mode power supply(SMP) and i allow all the filters to be fully discharged while switch on with load assuming the SMP is still carrying load but no mains input and all inductors should be allowed to be cooled before switching on the equipment(smp) and a current meter is connected to one terminal of the input mains from the joint box(jb) perticularly the hot cable to such loads and it can be numerious load (source of supply terminal to all the load on a current meter) is connected.€