I have a karcher power washer electric. residential use. WHen I turn the switch to on it immediately trips the curcuit breaker on the karcher plug. Out of the blue moon sometimes it works.

Which breaker does it trip? The MCB or the RCD?
If the former, the motor is taking too much current and that might be caused by shorted turns, a defective centrifugal switch (if fitted) or a shorted start or start/run capacitor.
If the unit has been standing for a while it could be the pump is frozen - some units provide access for a hexagon key so the pump can be turned manually a couple of revolutions before switching on the power.

If the RCD has tripped it is because there is an insulation breakdown and current is leaking to ground.

I fixed mine works good easy too watch my video step by step i put it on youtube
http://www.youtube.com/watch?v=UFXSLOBL0Mg

try to free the motor by turning it somehoe if its free replace running capacitor

1) Move wire to another same-size breaker.
http://waterheatertimer.org/How-to-replace-circuit-breaker.html

2) Understand electric service:
http://waterheatertimer.org/See-inside-main-breaker-box.html

3) Troubleshoot.
http://waterheatertimer.org/Troubleshoot-household-electricity.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

see the following steps: use the common sense God bless you see the diagram.
1. No power to the washer
Make sure there's power getting to the washer. Check for a tripped circuit breaker or a blown fuse. Check the wall socket for power. Sometimes the power cord disconnects at the washer, if this is the case, the wiring must be repaired or replaced.

2. Lid switch
The washing machine may not function if the lid switch is defective. The switch is usually located near the door. Replace if found defective.

Check and see if you are getting power to the on/off swith. These switches have been known to go bad. Also, check the Ground fault breaker in the plug. Let me know how you make out.

Ia the fuse blown? or the breaker tripped off? Must know if these are the problem. Plug in other small appliances in outlets in the kitchen to see if power is there. If not go to breaker box(usually in garage) check your sticker label and reset horizontal switch(curcuit breaker)back to on position. Note; sometimes these breakers can look like there on, if not sure switch off and on again. If you have power to outlets( INCLUDING WERE WASHER IS PLUGGED IN) you'r all set. Check the outlet where washer is plugged in, you may find a breaker built-in to it. The reset will be between upper & lower outlet, if so this needs to be pushed in.(most have tiny red button) Keep in mind the outlet might be defective. Never work on electrical untill you are sure that you have the right breaker OFF! If not sure call a professional! Good luck, best regards.

A few things to check, the GFI plug has tripped, the machine has safety switches under cover, could have gotten wet and froze. Take cover off, plug machine in, with a electrical tester start where power enters machine and test all switches till you find bad one.

Overload contition in circuit. Turn some stuff OFF.