Posted this here as it's not railway related

Can any of our resident experts on the above help with suppression of relay arcing please?

The problem relates to the twin cooling fans on our Sierra Cosworth. It is already acknowledged that the standard wiring and single relay fitted by Ford are not up to the job of powering the uprated cooling fans on the RS model, and they are a regular source of burnt out wiring! I have rewired them through separate relays each fitted with a 30A fuse which copes fine with running the fans ordinarily, but when they initially switch on the surge (I presume it's surge, or arcing across the relay contacts maybe?) is enough to occasionally blow the 30A fuse and hence no cooling.

Now thinking back to much older cars with points distributors they had a "condenser" fitted that was there to prevent excessive arcing across the contacts which would affect the ignition timing. I think this was capacitor-based, but my knowledge from there is limited. Can anyone with more knowledge of these things please suggest how I could minimise arcing across the relay terminals when the circuit is switched on?

Many Thanks


Paul

You're on the right lines I think!

I'm no expert on this, but a resistor and capacitor in series, across the terminals, should help. I found this page which gives some details including calculating values:

http://www.industrologic.com/mechrela.htm

So I reckon if we're looking at 12v, 30A the Capacitor needs to be 48v rated, and either 90, 15-30 or 0.1uF and the resistor should be not sure, 6-12 ohms or 47 ohms! Not really a lot of help is it?   

Hopefully someone with better knowledge will come along soon.

Glen

Hi Paul,
What about using a solid state switch like this?...
http://www.ebay.co.uk/itm/Solid-State-On-Off-Switch-With-LED-Indicator-12v-200Amp-/250821929303?pt=UK_Car_Accessories_Safety_Security&hash=item3a6626e957#ht_3108wt_938
Regards
Russ

Interesting suggestion, will look into that

Cheers Russ


Paul