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Kill Switch for Electric Motors

SAFETY FIRST!! Please read as no fish is worth dying for.

Kill Switch for Electric Motors

Postby Mindi » 02 Nov 2019, 11:16

Thought it might be helpful to discuss and draw up some notes for electric motor users who might think a kill switch is a sound idea. It is actually easy to do correctly, only skills needed are to crimp some slide terminals on wire. A bit of background to start. This is the standard kill switch from eBay for $9 or from a boat shop for $92. (My OMC dealer)
 It is "normally open" so that the current is normally not able to flow through it until you pull the safety peg and the spring button goes down...then it is closed. In a normal boat application, this newly closed circuit connects the ignition  to earth stopping the petrol motor dead. It is an indirect but effective solution.The same thing can't be applied to an electric trolling motor. You need to actually cut the electric power source off from the motor to stop it. this next bit is important.....why not buy a "normally closed" version of the kill switch and run the motor directly thru it so that the main power line goes through the kill button and gets cut when the tab is pulled. ??? Good question, and the reason is that the motor draws up to 40 amps and the kill switches are rated to a maximum of 10 amps and if you ran your motor directly through one all day you would at the least cook it but more likely set it alight. So using a "normally closed" kill switch doesnt work. and is dangerous.
What does work....? a simple relay. You buy a Narva 68044 Switching Relay ($16) and use the output pin on it which is Nornally ON (87...centre pin) to  run your motor as normal. ( on this relay 87 is Normally Closed"or "ON" and 87A is "Normally Open" or "OFF") . You take heavy wire from the battery to pin 30 to provide motor power to be passed through 87 on to the motor, this is it's normal state......and when the safety peg is pulled and the kill switch closes, an alarm signal current comes from the kill switch to pin 86, the relay switches the motor current away from 87 to 87A which is not connected to anything (this is why it is called a switching relay) ....the motor stops immediately. I actually cut pin 87A off with sidecutters.
Here is a crude amateur wiring diagram

This is where I have my relay mounted inside a battery box.

I hope this is helpful to anyone running an electric motor and thinking about a safety switch, it probably takes less effort to do than to document it. Apologies to any sparkies who accidentally read this.
Kill switch is totally sealed so can be mounted outside battery box but needs decent clip to attach to lifejacket or belt
Narva 68044 Switching Relay mounted inside battery box for protection $16
wiring diagram
standard normally open boat kill switchbefore mounting from eBay $9
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