Why is a relay needed for this switch-pic

This is what I did for my fuel kill. I had a reed switch wired where the #2 relay is.  That reed switch kept blowing - it would fuse and stay closed.

How is this second relay supposed to help? I was wondering because I'm still using power to the fuel pump to energize the coil in the second relay. That power then heads to the switch - isnt this the same as just wiring a reed switch where the #2 relay is?!

You mean the coil within the relay?

 [QUOTE] reed switches can supply a small amount of current, therefore using the reed to trigger the relay is like haveing a reed switch capable of sourcing 30 or 40 amps. [/QUOTE]

You mean a reed switch can handle a small amount of current?

 [QUOTE] therefore using the reed to trigger the relay is like haveing a reed switch capable of sourcing 30 or 40 amps  [/QUOTE]

I hear what your saying - but how is the reed switch now able to handle higher current.

From the diagram - I still see the switch getting power from the original fuel pump power wire. I don't get how this switch has not fused(I've used it 3 times as far) like the other reed switches I have used when I had a switch where the 2nd relay is located.

Its kinda hard to see in that diagram, but im assuming they are using 2 relays to cause a latching relay effect

The problem is reed switches must have small contacts to function, if the metal surface area is low, then more resistance is present, since power disipated in the device is I^2 * R, drawing 40 amps through a reed switch that offers just 1 ohm of resistance (actually higher than it would be) would cause around 1600 watts to be built up in the reed switch. That is what causes all the heat and fuses the switch.

By using a reed switch to trigger the relay it only needs to provide around 170 mA of current, if it still offers 1 ohm of resistance, total power consumed is about 29 mA, 54000 times less heat.

Its like you are using a small switch to flip a bigger switch.

I see what you are saying, just get rid of relay 2 and put the reed in there, that way it will still trigger relay 1, i would agree except if you do that not only will you blow the reed switch because of the current it is sourcing before relay 1 comes on, but if you do that and it didnt blow the reed the fuel pump will shut off as soon as you remove the magnet.

It is set up like that so that relay 2 can source current until relay  1 comes on, then relay 1 will stay on as long as it gets power (untill the vehicle is shut off.

If you are tring to save using a bunch of relays you could replace both of them with 1 SCR that is capable of handling that current, then just trigger the gate with the reed switch. I think thats what i would do.

Hope that helps.