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I'm thinking of buying some Edison-base screw-in miniature circuit breakers to replace some of the fuses in my house. There are some made by Connecticut Electric and some made by Cooper Bussmann.

Cooper Bussmann writes that their Edison-base mini breakers aren't for inductive loads. I don't know too much about electricity. What does "inductive" mean?

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up vote 7 down vote accepted

The simple answer is: Don't risk it and get ones that are rated for inductive loads. Here's the relevant Wikipedia article on power factor. Basically you have electrical impedance which consists of electrical resistance (which is opposition to movement of current created by resistors) and reactance (which is opposition to movement of current by all kinds of coils and by capacitors). In DC systems, the reactance is zero.

Actual reactance depends very much on the circuit itself and it can be zero and then we say that the circuit is resistive, it can be greater than zero and we say that the circuit is inductive and if it's lower than zero, we say that circuit is capacitive. By adding capacitors to inductive circuit we can make it less inductive, resistive and, if we add enough capacitors, capacitive. Same goes for other way around. If we add enough coils to capacitive circuit in the end we can get inductive circuit.

The problem here is that it's not uncommon to find device that doesn't provide enough data to easily determine if it's capacitive or inductive, what its power factor is and it can be difficult even when all that is known to calculate if the total load on the circuit breaker is resistive, capacitive or inductive. Another point is that the circuit-breakers linked don't provide (or at least I can't find) enough information to determine when an inductive load is too inductive for them.

So to be safe, just get circuit-breakers that can break an inductive load.

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It means things that consist mainly of an electric motor, or have a transformer.

Things that are OK:

  • Lights
  • Laptop / PC
  • Kettle (but check that it's not an induction heater one)

Things that are not OK:

  • Washing machine
  • Vacuum cleaner
  • 110v - 240v transformer
  • Fridge
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Thank you very much: that clarifies things. I wonder if you could please edit your answer to also mention: 1. Fluorescent lights. 2. Electric fan heaters. 3. What about 1000 W of lights, plus a vacuum cleaner, all together on one branch circuit? – unforgettableid Mar 30 '12 at 0:30
@unforgettableid, if it has an electric motor in it you cant use this part. If you think it might have an something in the not okay list you need to not use this part. If you start combining sets of appliances you would need to have a good idea of what they consider too inductive, but we are talking about possibly causing a fire, are you really saving that much money to risk it in that situation? – Kortuk Apr 17 '12 at 2:24
Note that some kettles will be induction heaters, which are highly inductive (as the name implies). That might not be a good example... – Kevin Vermeer Apr 17 '12 at 4:26
Laptop/PC will have a switching power supply that includes a transformer. – sharptooth Apr 17 '12 at 12:37
@sharptooth: The problematic transformers are the ones that are large and operate at 60Hz. A 15 amp breaker isn't going to have any problem killing power to a 1W transformer inside an alarm clock, nor will it have problems with a laptop's switching power supply that converts 60Hz to a high frequency before passing it through a transformer. – supercat Apr 17 '12 at 15:19

If one tries to use switch to interrupt current through an inductive load, the electrons will do whatever they have to do to keep flowing for a little while. If they have to jump through the air between the switch contacts, that's what they'll do. If the electrons have to work hard to keep flowing, they'll slow down pretty quickly, but they don't stop instantly.

If one has a circuit with two thousand of watts of light bulbs and a small 5W induction motor fan, the current flowing through the fan won't want to stop instantly, but the light bulbs will provide a pretty decent path for it. A little current may try to jump the switch, but not enough to do any damage.

On the other hand, suppose one plugs in a device that acts like a pure 10mH inductor in parallel with a one-watt light bulb. The inductor would draw tens of amps, and the light bulb, 10mA. If the breaker tries to open at a point when the inductor was drawing 40 amps, then 40 amps would have to keep flowing, at least for a little while. A one-watt light bulb isn't going to let 40 amps flow through very easily; the switch is apt to be a much easier path.

The big concern with the breaker is not to use it to interrupt loads which are primarily inductive. If a load is roughly half-inductive and half-resistive, the voltage required for the current that was flowing through the inductor to instead flow through the resistor will be roughly the amount that was required to push that same amount of current through when the circuit was powered on, i.e. the supply voltage.

Note, btw, that a breaker will be able to interrupt a combination of an 5A inductive load and a 15A resistive load far more easily than it could interrupt a 5A inductive load with no resistive load. If a breaker will only be opening in case of overload condition, a 15A breaker might reasonably-safely guard a 5A inductive load, if one could be reasonably certain that any other loads would be resistive. If there's a reason for the breaker to trip, that would mean there was enough resistive load to absorb the energy; if there isn't enough resistive load, the breaker shouldn't trip. I wouldn't want to rely upon things working so smoothly (e.g. someone might use some other switch to disconnect a big resistive load just as the breaker is tripping) but they should mostly be okay.

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