It all depends on where and what you are supplying. In hazardous locations you have to, the potential between neutral and earth is a problem. With floating supplies it should be done. Another advantage is that the circuit opens with each pole and in effect you have two in series. But on the other hand with the average installation it can be painful and a possibly hazard if it fails to only open the neutral.
2-pole circuit protection of any kind, breaker or fuse, applies for line -to- line voltages. Here in the states this is most commonly 208 and 480Vac. There are instances allowed(?) by the NEC whereby the neutral circuit can be opened in a line -to- neutral config.
Where load is connected between 2 phases of a 3-Phase supply, it is essential to protect it with a 2 pole CB so that both "hot" lines are opened under fault condition or fully isolated when switched off, (As when maintenance is to be performed on the load.) On a 1-phase supply where there is any possibility of line reversal, e.g. where supplied from a non polarized two pin plug, both lines must be protected - even when one line is Neutral, as, in that case it is not known which line is "hot". It could also be argued that, as there are 2 poles in series, the circuit may be broken more effectively, (i.e. 2 arc paths now in series), but this is not a normal consideration in practice.
Tony Firth, Electrical Eng., Quester Technology Inc., Fremont,CA
