Bonding of a bolted-together equipment rack located in a vehicle

Question

I have a custom built stainless steel equipment rack that is made up of many individual sections of stainless steel angle that are bolted together to form the complete rack. The stainless steel parts will not have any paint or finish applied.

The equipment rack will be mounted in a vehicle with the bottom sections of the rack bolted to the metal floor of the vehicle.

The equipment rack will have 12 Volt and 28 Volt equipment mounted on it.

My question.....

- Does a single wire connection from the vehicle's Main Earth Terminal (MET) to the equipment rack meet the protective bonding requirements?

I am hoping that I do not need to have a separate wire from the MET to each individual piece of steel angle, or a 'strap' linking the bolted-joints.

broadgage · Accepted Answer

In my view a single bonding connection to the whole rack would suffice, and in view of the extra low voltage, even that might not be required.

mapj1 · Answer

If you are not sure how many milliohms to worry about, and actually want a figure, you can estimate if you can decide what the worst credible fault is - perhaps a short from a battery supply fused at 10A would need to drop no more than a few % of the battery voltage at 10A, to ensure a fast and reliable 'pop' with a few times 10A, without overheating the bonded part too much in that time. 
 So for an item that could credibly short to a cable fused at 100A, the bonding requirement needs to be that much better than if the requirement was to operate a 10A fuse. If you can be sure that the fault condition is incredible - if the insulation chafed off and then the wire snapped it still would not reach, sort of thinking, then you do not need to worry about that current path at all. 
 In vehicle battery systems where the voltages are lower and the currents higher than mains practice, the impedance can easily be beyond the lower limits of a normal meter, and then not worth trying to measure, rather things are just bolted up nice and tight onto clean metal and the resistance calculated/estimted. Usually fixing metal work gives you lots of cross-sectional area, and even in a large vehicle the distances are quite short. 
 (For an extreme example a motorized tank turret I think was 200-300A at 24-28V - so to blow the 500A fuse fast needed a reasonable multiple of that, say about 2000- 3000A , so that needed (and had) a return path of less than ten milliohms or so. (1mm2 copper is 16-19 milliohms per metre, but you should also consider the adiabatic heating of that.) The tank cables were something like 35 or 50mm2 - I am quite sure your case is much less extreme !! These figures are not exactly right, being dragged from memories of a conversation many years ago with someone who worked on the controllers for the things.) 
 Note that your earth wire may need to be reasonably chunky for mechanical strength, even if low current handling, unless protected. The normal 7671 rules used at 230V are not really what you need but there is some read-accross of the ideas. 
 Mike.

Newfutile · Answer

I can't see any bonding is needed In the usual sense of the word,we only bond to keep voltages from being unequal between simultaneously accessible conductive parts. As you using extra low voltage this is most likely not required,unless I'm missing something or it's for other reasons as stated above

AJJewsbury · Answer

Sparkingchip said: is there such a thing? 
 If there's a 230V system with an earthing arrangement there must be as a matter of principle - even if in practice it's just a spade terminal on the output of an inverter. 
 - Andy.

Bonding of a bolted-together equipment rack located in a vehicle

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