You might have the neutral wire connected to the fan switch instead of the hot wire going to the fan. The switch for the fan disconnects thus the neutral for both the fan and lights instead of just cutting off the hot wire to the fan, alone. (Generally, the fan and the lights share the same neutral wire returning to the circuit breaker box, but have their own designated hot wires carrying current that ultimately comes from the circuit breaker box. Even if that current for both fan and lights at one point comes from the same hot wire, it needs to be spliced at some point to provide seperate hot wires for the fan and lights.)

This is a very dangerous wiring scheme, which should be corrected no matter what. It means that the hot wire to the fan is not switched and current is constantly running through the fan even when it is turned "off." The switch should always shut off the hot wire, not the neutral wire. If current is constantly going through the fan unit, it can seriously shock someone if they work on the fan with the switch turned off, wrongly thinking that no current is going to the fan because the switch is off.

The current should also be stopped at the switch BEFORE it runs through the fan, not after it returns from the switch, for the same reasons: you don't want someone to work on the fan with the switch "off," thinking that there is no current going to the fan because the switch is off. Even if the switch is after the appliance on the circuit, the hot wire should not be connected to the appliance directly in the appliance box. It should continue its run to the switch, be cut off there, and then connected to the hot wire of the fan AFTER it returns from the switch.

Another possibility is that they connected the hot wire for the fan to one pole/screw on the swicth, and the neutral to the other, completing the circuit in the switch instead of through the appliance. This also is a very dangerous wiring scheme for much the same reasons. Only the hot wires should be connected to the switch, and the neutral wires connected directly to the appliances/outlets. The switch then interrupts only the hot wire and truly does turn the appliance/outlet off (no voltage going through the unit) when it is switched "off."

[This message has been edited by Lawrence (edited December 27, 2003).]

Since you said it dosn't always happen you must have a bad connection somewhere. The wires for the fan are shorting to the wires for the lights. It could be in the switch box or the fan box.