7 day timer and Inductance current

[Chris Crinkle]

I have a 7 day timer, with a max load of 13A at 240V. However reading the instructions its states there is an inductance current of 3A. What does this mean ?

[John Williamson]

If the load isn't resistive like a heater element, then the permitted load is reduced.

Inductive loads such as the motors used in fridges or washing machines can quite easily generate sparks when the power is switched off that in an etreme case can weld the contacts together or damage them over time so that they no longer make a low resistance connection. The problem can be minimised by bridging the contacts with a capacitor, but then you get problems with the permanent leakage current into the load, with the safety problems that can cause.

[fred]

In article , Chris Crinkle writes

It means that the 13A rating applies only to purely resistive (heating) loads.

An inductive load is something like a motor or an old style (inductive ballast) fluorescent tube lamp and when a circuit containing something like that it disconnected, voltages are generated that could cause arcing across the opening switch contacts. The inductive rating is the maximum current that can be switched on that kind of load without severely limiting the life of the contacts due to arc wear or indeed welding them together.

Similar restrictions apply to loads with large inrush currents such as computer power supplies.

[John Rumm]

It means that if you are switching a load that is inductive in nature rather than simply resistive, then the maximum capacity is reduced to

3A. (inductive loads cause big switching transients that have a habit of welding the relay contacts in the programmer!)

[harryagain]

Inductive loads (motors, coils etc.) have stored energy in their coils which means the current you have to break can be much higher than is on the rating plate.

So any switch rated for a non-inductive load (lights, heater) will be down rated for inductive loads. Some are not suitable at all for inductive loads.

[Brian Gaff]

Yes you get a storage effect much like a magneto.

Brian

[Cursitor Doom]

I don't really understand why peeps here are describing heaters as non- inductive loads. They most certainly ARE inductive; not so much as motors, but significantly inductive nevertheless. Reason being the resistance wire used in them is bunched up in coils to save space and where you have a coil you have inductance. In fact even a straight wire has a tiny amount of inductance but that's another story. Coils store energy in the form of a magnetic field and when the current is switched off, this field collapses generating a back EMF (a voltage spike). This can be several thousand volts in some cases, causing arcing of the switch terminals and shortening their life. You therefore have to 'de-rate' the current to allow for this.

[harryagain]

The inductive effect is insignificant in electric heaters with a few dozen turns of conductor in air.

Significant inductors have hundreds of turns of wire wound onto iron cores which store significant energy in the form of a changing magnetic field.