Lightning Protection for 12V, 500mA system protection

Depends on how the lighting is taking it out. Is it more of an EMI/EMP type deal, or an electrical spike on the power feed?
A good ground strap or a surge protector should do it.

If it's more like a direct hit, something a little more heavy duty, like a Lighting Surge Protector on the Mains feed, might help.

Surge protection on the power is in place. Please see where problem lies above.

You said:
"the petrol pump monitoring system which is frequently struck by lightning...
We find that the controller box is frequently damaged by lightning surges."

How is it being hit by lighting?
- Is it mounted on a 40 foot pole? Side of a building?
- Are these "direct hits", or "ground shocks" from near misses?

A few details, please.
From what little I know of the situation at the moment, your fuse idea might be the way to go.

did you use shielded cables and metal boxes to put the pump in?
normally - if these are earthed, it should solve your problem

Hi coral47

The Petrol Pumps are mounted below and above ground. Pipes run from petrol tanks below ground to the dispensing units from where a hose is used to fill up the cars.  We suspect that they are "ground shocks" from near misses or from a large cell phone mast not too far away that attracts the lightning and conducts it to ground.

The controller units are inside the nearby office where the petrol attendants deposit the cash collected etc.

Lightning can also hit the pumps directly and damage the controller unit this way too.

The cables are shielded and earthed nobus.

What your missing in understanding the problem is the reverse nature of electricity in that all of the electrons are in the negative side of the circuit (meaning ground).  With lightning, in the last split second before the strike, voltages of up to 1700 volts per centimeter rush through the ground to the point of impact and, what damages equipment is the difference between two (or more) ground points.  What you need to do is to have the attendants end completely isolated from ground with a large, shielding conductor connecting it to the pump.

Tx 4 suggestion.  
"What you need to do is to have the attendants end completely isolated from ground with a large, shielding conductor connecting it to the pump."
Please elaborate and perhaps you can direct us to the product we may consider.

or setup a lightning rod, taking the discharges away

But here is this tall cell phone mast in the area that should be doing this...?

What you want is to have one (and only one) earth ground connection for the entire circuit.  Then, any voltage surge will cause everything to float together.  When there are two grounds, you wind up dissipating the voltage differential through your equipment.
Its OK to have a protective ground on the AC side of a power supply; but, it should not be connected to the logic ground on the DC side which can be checked with a multimeter.

This is presumably good for ground strikes, but if the actual pump is struck how would it protect?

Ground all pump casings and use insulated wiring that connect them all with common grounds.
All pump casings and their insulated wiring need to be connected with multiple grounds.

Also consider:
DC 12V Lightning Proof Power Surge Protectors

such as:

http://www.securitycamera2000.com/products/DC-12V-Lightning-Proof-Power-Surge-Protector-for-CCTV-System.html

After a point, it is impossible to protect things from a direct strike; but, I think your primary issue is two ground points and the resulting voltage difference of nearby strikes.  The pump is definitely one ground point and I'll bet there is a second ground where the AC enters the station.  If you move the AC ground stake as close as you can to the run between the pump and the station and run a wire from it to the pump, you'll virtually eliminate the differential which will stop the failures. 4 gauge aluminum wire should work fine.

>>  But here is this tall cell phone mast in the area   <<  how high - and how far away?

I've established that the 2 wire control system is actually a TTL DC 5 or 12 V system depending on the type of petrol pump used.
Please review answers above in light of this info and suggested protection devices.

Please provide additional info on TTL as I'm not familiar with this protocol.

Is it important that the foil sheath surrounding the data cable is earthed at each end? I've noticed that this has not been earthed.

Just to reiterate - there is a cell phone mast near this particular garage and the claim is that seeing as though this mast would be very well earthed and the petrol pumps perhaps not as well earthed that this could be causing the frequent strikes.
If this is the case please suggest steps to rectify.

normally, you earth a device or cable at one end only
best near where the strikes occur
normally, a n earthed mast near the site should protect the site  from strikes

Your problem is not that you have poor grounding; it is that you have two good grounds that are several feet apart and, as I said in my first post, are dissipating up to 1700 volts per centimeter times the distance between them.
Wherever the AC mains enter the building is a ground wire running to a stake.  The Pump itself is another ground.  Measure the distance between the two and do the math.
Move the AC mains ground stake as close as you can to the pump.  Make sure the pump and that stake are thoroughly connected to each other, electrically.
That is what will minimize the damage; but, you must understand that nothing can make you lightning proof as the energy is far too high.

DavisMcCarn: It makes a lot of sense - need to investigate this thank you.
What gauge of cable would you suggest to join the 2 earths?
Can we use an inexpensive multimeter in continuity mode or resistance mode to estimate and measure the two earth distance?

nobus: The height is not great - a pretty standard mast height for 3G masts in South Africa - will have to establish the distance.

 kode99: Any feedback on this: ID: 39610598

"Is it important that the foil sheath surrounding the data cable is earthed at each end?"

Yes, this foil sheath surrounding the data cable must be earthed to the casing at both ends and the casing must be connected to earth rods.

It is impossible to protect it from a direct strike:
Read this:
http://stormhighway.com/protection.php

Also connect a Power Surge Protector to the power supply of both the AC supply and the DC supply.

tx. DavisMcCarn.

" if its a long distance 2 gauge or copper would be better." I would have though the longer the distance the thinker the cable recommended to reduce resistance effect?
Pls can you give these diameters  in metric.

2 gauge is larger than 4 gauge.
http://en.wikipedia.org/wiki/Standard_wire_gauge