Which black wire is neutral and which is switched live?

[Davechip]

Replacing ceiling light fitting.

Two cables, each with a black wire, presumably one for mains and one for switch.
Neither has any indication (red tag etc.) which is which.

I don't have the equipment that checks which is live.

Is there any danger in wiring it up one way, seeing if it works, and if not then swapping the black wires round???

 

[jj4091]

As long as you do not connect either wire to the earth terminal then the light should work which ever way you connect it.

 

[ericmark]

The problem is where you have more than two black wires then you have to work out which is which.

Standard ceiling rose will have four connections.
Earth clamp.
Permanent line centre.
Neutral outer.
Switch line outer.
Theory with a BA22d bayonet bulb holder which outer goes to which terminal does not matter. With ES27 Edison Screw the line should be centre but it will still work other way around.

With three black wires you have the following possibilities.
1) Connecting neutral in to neutral out next lamp all works correct.
2) Neutral in to switched line will blow fuse or trip when switched on.
3) Switched line to neutral out light will only work when another light is also used and will likely be dim.

Clearly you want to avoid number 2 if possible but unlikely to cause any permanent damage with normal switch but will likely destroy a dimmer switch.

 

[PrenticeBoyofDerry]

I don't have the equipment that checks which is live.

Should be the first thing you obtain if undertaking electrical work of any description. As this would suggest that prior to working on the circuit, you did not safely isolate the circuit and prove circuit dead.

Is there any danger in wiring it up one way, seeing if it works, and if not then swapping the black wires round???

It will work either way but on certain lampholders it will present a danger if connected in reverse and a this will create a potential hazard, so I would recommend that you ID the perm live and connect correctly, this can be done by means of either a voltage indicator(two pole) or someone competent to ID it for you, then mark/sleeve/tag the perm live for future reference.

As long as you do not connect either wire to the earth terminal then the light should work which ever way you connect it.

As Eric has mentioned it may work but on certain lampholders it is dangerous to have them connected wrongly, I suggest the OP either acquires a voltage indicator or asks for the service of someone who possesses one to identify it and mark up accordingly.