I was looking to get a timer for a electric heating deicing strip which draws about 13 amps and saw some rated up to 15amps resistance or 10 amps tungsten, I also saw ones marked 15 amps resistance or 5 amps lighting.
I take it as tungsten is the traditional lighting and the 2 terms are interchangeable.
But have questions:
1 - I know power can be taken as resistance or impedance (though I’m not sure this is the correct term), but is not lighting resistance?
2 - Use of a electric heat strip certainly is not lighting but is it close enough that I should look for a different switch?
3 - Why the restricting on lighting? The only thing I could come up with is lighting is pretty much a constant use of power while other devices have a initial surge current then a lesser running current, and while the switch can handle the 15a for a brief time it is not meant to run continuously like that (meaning it’s a bad idea to use it on this deicing strip).
Tungsten, and indeed most or all pure elemental metals (as opposed to alloys), has an electrical resistance that is roughly proportional to absolute temperature - which changes by nearly an order of magnitude from the off state to the on state for an incandescent bulb. Thus a tungsten lamp that draws 10 amps when hot will draw nearly 100 when it is first turned on at room temperature (unless the resistance in the supply wires drops significant voltage too under this condition, which is fairly likely).
If you have ever heard of nichrome heating element wires, you may be interested to hear that nichrome (which is similar to the metal from which US 5 cent coins are made) was designed to have a nearly constant resistance over different temperatures, which gets away from this problem. But it can’t work at high enough temperatures for incandescent lighting.
I think that’s because the resistance of tungsten goes up as it heats up. So until the filament heats up to the normal operating temperature, it draws much more than the rated power.
What the others said. I’ll add that resistance and impedance are similar, but not identical. Impedance is basically the fancy form of resistance, with all the optional extras. These extras only become apparent with alternating current, though. For direct current, impedance and resistance are the same.
With alternating current, impedance takes into account momentary energy storage by components in the electrical circuit. Resistance does not. Therefore, you can end up with different values for things like “power” when calculating impedance versus resistance.
As mentioned by others, an incandescent lamp has a pretty big inrush current due to the filament’s low resistance when it is cold. The resistance is much higher (and the current is much lower) when the filament has reached its steady state operating temperature.
Handling a high inrush current can be a challenge for the controller’s switching element. If the switching element is a mechanical relay, the contacts must be designed to handle the high current during contact bounce. If the contacts are not designed for this, they can fuse/weld together. (The high current during inrush will create hot spots and localized melting on the contacts, and the contact alloy must be able to withstand it without welding together. AgSnO[sub]2[/sub] contacts, for example, can handle higher inrush currents than AgNi contacts.) If the switching element is solid-state, the FET or Triac must be designed to handle the high levels of current density during inrush, assuming a zero-crossing circuit is not employed.