# Time factor in specifying rating of electrical components

Ratings of electrical components like switches and sockets are marked in Amperes. Which means above this current the component is likely to fail (melt, burn, etc.), if run over some "considerable" amount of time. How do they determine this "considerable" amount of time?

The amount of heat generated (temperature rise) is proportional to the time of current flow. So even if one runs a current below the specified rating, over a long period of time, even accounting for heat losses, it would reach the critical temperature of failure of the component. For e.g. a switch is rated for 10A. Could I or should I run 9A current over a long period of time? Nowhere do they mention how much time.

• You forgot one critical part of the equation: Cooling. If the part is able to get rid of the heat as quickly as it heats up (that is, it reaches thermal equilibrium), it won't heat up further. Commented Aug 12, 2021 at 17:51
• If potted or in the vacuum of space, there is (little to no) cooling so that would be true. But air is a surprisingly good thermal sink, and will keep a 10A switch cool at 9A. Commented Aug 12, 2021 at 18:00
• @rdtsc Do you mean the surface temperature of the switch, or the temperature deep inside the switch? Isn't it the temperature of the weakest link down inside the switch what matters, and if the thermal mass of that weakest link is small then won't it have little effect on the surface temperature? I think you are treating the switch as a homogeneous block of conducting stuff. Commented Aug 12, 2021 at 19:54
• ontherocks, Perhaps you could improve your question. It seems almost as though you'd like the means by which good quality manufacturers make theoretical predictions, which lead to design and manufacturing requirements, which are then validated against the original theoretical expectations to close the loop and then put out a product with well-supported specifications. But I'm not sure. (There is always good theory behind all good designs.) Am I off-base to think that's where you are headed?
– jonk
Commented Aug 13, 2021 at 9:01

If the manufacturer does not specify a time then you must assume that when you exceed the manufacturer's specified limits that the device will fail, or at least degrade, instantly. There is no guaranteed "considerable amount of time" if it is not in the datasheet.

You need to take into account the cooling of the part being measured, whether by radiation, conduction or convection.

The manufacturer will determine the likely worst case situation in which their component will be used. Then the maximum current is OK if the component is still at an acceptable temperature, once the equilibrium is reached between heating and cooling.

• Don't you need to ensure that all parts of the component are still within the specified temperature range? If a thin wire deep in the heart of the component is glowing red it doesn't matter that the outside is a room temperature, does it? Commented Aug 12, 2021 at 19:51
• @ElliotAlderson That's going to be up to the manufacturer, taking into account any national or international standards they need to comply with.. Commented Aug 12, 2021 at 20:32

You can generally run 9A through a 10A switch indefinitely if your conditions are no more severe than those the switch is specified for and are used in the intended manner.

Those conditions can vary widely, for example, two 10A rocker switches I look at, at random, one had a maximum operating temperature of 55 degrees C and the other a maximum of 85 degrees C.

Now, if you take an ordinary 10A switch and attach it with thin leads and put it in a vacuum so it can’t cool itself it will fail quickly, because that’s not the intended application.

You may also wish to derate the switch (use it at less than rated current) so that the life is more than the minimum specified in the datasheet. Usually the mechanical life (life when switching little or no current) is at least 10x greater than the electrical life at full load. By using a 16A rated switch for 9A your product may last longer and run cooler.