The KJD17B is a combination on-of button and self-latching solenoid that acts as a low voltage drop out.

The symptoms you describe are consistent with it, or the wiring to it, not performing as expected. Before going in too deep you can test the switch on its own controlling a spare lamp or something to convince yourself it is OK, - it needs the neutral connection to hold on, and if that is not there for some reason...

Test circuit - note that the actual contactor part is wired double pole on  many models, but the self holding relay part is still the same - the holding coil  is in parallel with the load once it is energised, and that keeps it on until the supply is interrupted. The start and self holding contact are shown here as two in parallel, and you can add extra switches like that but in the unit they are not really distinct - instead the 'start'  button mechanically nudges the same contact that the coil pulls on.

Of course you may already know this,  in which case apologies.

Note that the not so helpful data sheet implies it is just a switch also shows that the link to coil neutral side needs to be completed  (optional ohne und mit 'spulenanshluss')

Mike.

Hi Mike,

I took the old switch to pieces and its fairly straight forward to understand how it works. It was knackered, terminals burnt and not contacting fully.

The new switch doesn't work as required. You can see in the images I tested the continuity and voltage across different test points on the switch whilst live. Advanced wooden brush was used to hold the lever down that the coil should hold in place once powered up by normally pushing the green on button to push down the lever, make the terminals contact and self power the coil until the lever is forced up manually overcoming the magnetic field holding the lever down.

230V between phase (terminal 23) and neutral (terminal 14)
230V between phase and the neutral wire to the coil.

230V between all other points tested, i.e main phase and neutral inputs, across the thermal cut out etc. 

0V across the coil yet there is continuity. This was not giving even the slightest notable force holding the lever down. As soon as any pressure was removed holding it down gravity took over, lifting the lever and breaking the contact. Now since gravity is a very weak force at those levels that to me would suggest that the magnetic force that should be holding the lever down should easily overcome gravity. I presume there should be some voltage across the coil? 

Thanks

Neil    

yep the coil is normally (perhaps via another contact ) in parallel with the load - and the 230V accross the coil holds in and replaces the finger or wooden brush on the trigger once the load is started. But clearly not for you - So where do the wires to the 2 ends of the coil actually going ?

yep the coil is normally (perhaps via another contact ) in parallel with the load - and the 230V accross the coil holds in and replaces the finger or wooden brush on the trigger once the load is started. But clearly not for you - So where do the wires to the 2 ends of the coil actually going ?

There are two thin pieces of wire that connect to the copper coil at each and the opposite ends run down and contact the live and neutral terminals.

I took off an old switch from another tool and tested it and it works fine on the tool that was faulty so I know that I have been supplied a faulty new switch now.