Measuring peak power for rating plate

I can't help wondering if the term "peak power" is well enough defined to put on a rating plate.

I once researched why a particular piece of aircraft equipment occasionally blew its fuse when in an aircraft but never in the lab and discovered that, depending on just when in the cycle it was turned on, it could draw anywhere from 1 to 100 times its steady state current at switch on.

I would not know what value to use for its peak power.

If it were say a 500 W tungsten halogen lamp (I know, don't digress!), what figure would you put on the rating plate?


Regards


BOD

Assuming the user wasn't permitted to fit a higher wattage lamp in the fitting, I'd say 500W?

I would expect that peak power would have to relate to steady state conditions, else for motors you would need to consider starting conditions and maybe locked rotor power.


If you consider why we might need to state peak power at all, then I guess it becomes clearer whether or not we need to worry about short term / in-rush conditions when writing a number on the name plate.  Also, you could always state the required protective device in addition to load characteristics which in your case might be more useful.

Depends rather what the information is for. In terms of rating plates 'peak' means in use but with with all the options enables - so all the rings on a cooker for example, or a motor running at max speed. 

The term 'inrush' is normally applied to the current that flows during start up of a motor, or charging input capacitors of a power supply.  If you really mean to quantify inrush then both current and duration are relevant, and as above if the inrush is only  fraction of a cycle then as noted above, the exact instant of switch on matters - flat capacitors have highest inrush if connected to the mains at the peak voltage, while the worst time to connect an inductor with no pre-magnetisation is near a voltage zero-crossing - this would normally be the current maximum, and gives the greatest transient to sort out.


You need a scope and something to synchronize your switching electronics to the mains cycle if you want to measure this repeatedly, or you need to capture many transients and note the spread of results.


As inrush can be many times higher than normal loads, at least for larger loads, it is normal to design in some sort of inrush limit to define the worst case current. This may be as simple as a resistor that is bypassed by relay after a delay, or a series thermistor whose resistance falls as it warms up.

Also if equipment is likely to be run on an inverter, it may see the equivalent of an inrush surge on every cycle if the waveform is stepped.

If you want to estimate the inrush for a halogen lamp, measure the resistance with  a meter when cold, and expect to see about 10% of the resistance when hot.

As MIke says, we need to know what "peak power" we are talking about (or more importantly, which "peak power" the OP requires):

• peak rms power (rms current multiplied by rms voltage - or VA)

• peak rms real power (rms current multiplied by rms voltage, taking into account power factor)

• peak average power (true average power over a given time)

• peak power, as in the maximum peak wattage the power wave reaches as the power varies over tim

Usually for appliances, the appliance will be set up for the maximum load conditions it would normally expect (e.g. washing machine will be on tumble + heating on hot wash) and the rms power or average power is measured - with the supply voltage being either the nominal voltage (if only one voltage is stated on the plate), or the worst-case voltage in a range if a range is quoted.


Some product standards require a maximum rms load current rather than a power to be stated on the rating plate, though. In this case, for example, an SMPSU quoted 80-230 V nominal would have the rms input current measured with a supply voltage of 80 V as this would be higher than the current at 230 V.

Hi, thanks for the replies.


This is for a lab system.  Mostly rack equipment, control cabinet with switched-mode PSUs and electronics.


I'm assuming equipment rating plates usually display peak RMS power?  The nominal operating voltage (230V AC) and frequency (50Hz) of the supply at the installation location have been provided so I presume no need to test at the lower limits of the switched-mode PSUs?


Parts of the system fall under the Machinery Directive.

For the equipment falling under the Machinery Directive, the full load current is required to be displayed on any enclosure that receives an incoming power supply to the Machinery as defined (BS EN 60204-1 clause 16.4).


BS EN 61010-1 (safety requirements for equipment for measurement, control and laboratory use) permits either power or current to be stated.


For electronic converters (power supplies), BS EN 62477-1 is not specific about whether the information is put on a label or in the instructions or product specification documentation, but the maximum nominal rms input current is required to be stated for correct selection.


It's important you determine which standards apply to the pieces of equipment, so you can comply with the specific marking requirements of the standards (particularly for Machinery Directive where this applies).

Many thanks for your reply Graham.  Both the BS EN 60204-1 and BS EN 61010-1 would apply to the system.


Is there any guidance on the type of meter to use for the peak power measurement?


I'm considering RI-F500 power meters (RI-F500.pdf) with 0.5s CTs, and using the peak demand logging function with all system modes operated?