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What does 230/400V mean?

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  • pricekitty4427
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Posted 28 April 2017 - 07:11 PM

I've noticed that some of my circuit breakers are 230/400V. 


I understand what voltage is, but I don't understand what it means when two different voltages are mentioned.


Any help?


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Posted 28 April 2017 - 08:16 PM

I'm not an electrician, and I haven't looked it up, so I could be wrong. But I'd say it is explained as follows:

There are two different ratings, one for 230 V and one for 400 V. There might be a good reason for having a 230 V rating as well as a 400 V rating, as a 400 V rated circuit breaker is obviously fine at 230 V, but the best I can come up with is just to make it clear that it is normal to use it for 230 V circuits.

The 400 V probably relates to 3-phase power. With a 3-phase system, there are three live circuits ("phases") that are all 230 V from neutral (and all have their own circuit breaker, but generally combined into a unit so that they all switch at once). Each phase is 120 degrees out of phase with each other, which means that there is a voltage from any phase to any other phase. That voltage is 400 V (= 230 x sqrt(3)). Some devices apply a load from each phase to each other phase (as opposed to being from each phase to neutral), and therefore a circuit breaker on a 3-phase system must be able to deal with 400 V. Imagine a short from one phase to the next, causing the circuit breakers to open. However, because they're real devices they won't open at exactly the same time – and therefore the first one to open must interrupt the whole 400 V (which is way harder than merely blocking 400 V, because the current could be several thousand amps).


So my conclusion is basically that the 230 V is there so that no one thinks it is not intended for normal systems, and the 400 V means it is also rated for use with 3-phase systems.

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