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Electrical Repair - Fusebox/ Sleepout

I have a pre-wired sleepout that has all electrical working. Currently connected to an extension from the house and all working fine. Also connections to main house wires are out and accessible. I have got a sparky to run a new fresh cable to the house through a underground pipe (previously used for a spa - dragged old wires out with new and resealed). We connected one plug to this new wire on the sleep-out side to check the connection - it works for a moment, but seems to throw the fuse box out and the house turns out. The sparky has gone overseas so can no longer diagnose the issue and fix for me. Need a sparky to diagnose the problem and find a fix.