shower shorting out?

when pulling the cord to turn the shower unit on which is attached to the wall which provides power to the shower unit, the red light on the wall box goes red as normal to indicate the shower has power and is ready to use.

but once the power button on the shower unit itself is pressed to start the shower the red button on the wall with the cord goes off so the shower doesnt start.

shower details are as follows

redring expession 500