ELCB Issues

Hi,
I have stripped out the old control system from a Denford Easimill and
replaced with Mach3 via a Bob Campbell break out board. It works
beautifully but from the start I had a problem with the machine
dropping out the ELCB on the mains supply. The machine is plugged into
a ring main which is fed via an earth leakage circuit breaker from our
consumer unit (I should say that I am in the UK). I found that if I
Estopped Mach3, this drops a relay connected to the change pump which
takes the power off the stepper power supply. Giving it a minute or two
allowed me to isolate the machine from the mains without the ELCB
dropping.....more often than not. Isolating immedaitely after Estop
seemed more likely to drop the ELCB. I have opeated this way since last
summer but just now I can't reset without losing the ELCB. I have the
machine powered up but in Estop mode, pressing reset powers the relay
via the charge pump output of the break out board which switches 240V
onto the stepper power supply and the inverter which is controling
spindle speed.
I know what the ELCB does but what is it that is tripping it? Any help
or hints greatly appreciated.
Ian