What does "10 (4) A" mean?

[keithfre]

I'm looking at room thermostats. The switching contacts are rated at "10 (4) A" or "10 (2) A". What does the number in brackets mean? Is it 10 amps, 4 or 2 contacts respectively?

Update: I've just noticed a reference to "Resistive 10A, inductive 4A". What does that mean in practical terms? Would the thermostat be suitable for switching a heater drawing 9 amps?

[Malc]

Does it possibly mean 10a resistive, 4 inductive. The difference is that an inductor is a coil of wire and produces a magnetic field. When you switch off the current, the magnetic field collapses. The rate of change of the flux causes a big back voltage/current spike that causes the contacts to erode. That doesn't happen with resistive loads.

If your heater draws 9 amps, it is certainly inductive and would need probably higher rated contacts.

I can see a thermostat having 2 contacts, but not 4.  There should be a pair of input connections and a pair of output (load) connections.

[Steven B]

The number in brackets will be the inductive loading for the switch. Inductive loads include motors (fridges, compressors etc), transformers and fluorescent tubes.

Despite the coils, heaters are a resistive load rather than inductive.

You shouldn't have any problems running a 9A heater via a 10A switch.

Happy modelling.

Steven B.

[keithfre]

Despite the coils, heaters are a resistive load rather than inductive.
Steven B.

Thanks, Steven. I've done a bit of reading and it confirms that a coil needs to have an iron core in order to be inductive (as you say, motors and transformers).

[Malc]

Despite the coils, heaters are a resistive load rather than inductive.
Steven B.

Thanks, Steven. I've done a bit of reading and it confirms that a coil needs to have an iron core in order to be inductive (as you say, motors and transformers).

Inductors do not necessarily have to possess a core and a coil of wire is inductive. Try pulling the mains plug out and see if you get a spark.

[keithfre]

Inductors do not necessarily have to possess a core and a coil of wire is inductive. Try pulling the mains plug out and see if you get a spark.

Sure, but without a core the inductance will not be enough to turn it into an inductive load. I read that with an inductive load the current waveform lags behind the voltage waveform, so the voltage peaks and current peaks are not in phase. If the peaks are in phase the load is resistive.