Battery Isolator Switch

[megadodo]

Trust you guys to look at things "sideways"!

[PaulF2B]

Hi, Just wanted to add an explanation on why you put the switch on the –ve side.

 

Although as stated above, this is a circuit and the current out should equal the current back, this is only true when the motor is already spinning and there is steady current flowing.

 

The issue is at the instant (time 0) when the starter switch goes on, the initial inrush current can be very high flowing from the battery to the motor to create the magnetic flux and start to move the motors armature. So this energy used to do this will be in the form of a Spike or pulse leaving the +ve terminal and going to the motor to be dissipated.

 

It is this short spike or the back EMF from the motor that can damage the isolator switch if it is places in the +ve side.

 

If you look at a battery it may be rated for CCA (Cold Cranking Amps) at say 280A but you would never be able to run a steady 280A through a cars electrics without melting the wiring, but it can provide that initial momentary surge of energy required to get the starter motor moving on a cold morning.

Hope this helps explain why you should fit the isolator in the negative.

Regards

Paul

[Guest peter2b2002]

i must be lucky as i fitted a key switch +ve 9 years ago the key has just started to got a bit slack so i have another key switch to fit,

peter2b

[Guest chris brown]

Not really Peter providing you just use it when no current is flowing (engine off) you should have no problem. The only thing is they can wear in the plastic bits. As for fitting in pos or neg line is personal choice as there is exactly the same current in both lines, you can't get amps (electrons) out of the pos side of the battery without having the same current flowing back in the neg side and no there isn't an initial kick electricity flows in a circuit (copper) at about 0.7 the speed of light.