If the rack contained telecoms or IT equipment with signal/data interconnections you might want to bond everything regardless to from a mesh bonding network - but that's more a matter of keeping various equipment's idea of Earth as similar as possible to reduce the risk of earth referenced data signals being corrupted - i.e. for functional rather than for electrical safety (protective) reasons.

   - Andy.

Steve R said:

vehicle's Main Earth Terminal (MET)

is there such a thing?

Isn't is called the "chassis"?

I know I’m getting old, but the first car radio I installed could be switched on the rear for installation in either a positive or negative ground vehicle, 

Other than the antenna and speaker, the radio only required one wired connection, the vehicle bodywork completed the circuit.

Things have changed, but not that much.

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.

Lost in the rambling mess this forum becomes, the original poster said there isn’t any 240 Vac on the rack, but I’m still unsure if the 12 and 28 volt equipment is AC or D.C.?

“Note - I have just noticed that I didn't mentioned there was 240 Vac in the vehicle. However, there is no 240 Vac equipment or cabling in/on the equipment rack.”

A full description of the electrical equipment and electrical installations within the vehicle might clarify things, as there seems to be several electrical installations all within the same vehicle, as can happen.

Good evening, 

The 28V and 12V equipment is DC.

The rack has only 28V DC equipment mounted on it.

Elsewhere in the vehicle we have various items of equipment that are 240 Vac. 

All equipment can derive supply from a battery pack mounted on the vehicle (via inverter), or from an external 240 Vac supply that's plugged in to an external connector.

Steve 

an external 240 Vac supply that's plugged in to an external connector.

Ah, 'shore power'. That stuffs things quite a bit, and can lead to odd things like shocks between the vehicle and the ground upon which it stands.

A broken cpc in the shore power tether is a moderately common and credible occurrence, especially on leads that get rolled in and out a lot. So if you have mains equipment (and not much does not) with internal mains filter with comparable capacitance  LE and NE can leave the chassis floating at about half mains voltage relative to terra firma, a nasty surprise for anyone grabbing the door to get in..

relying on the external supply for earthing can be a bit fraught, and for low power it may be worth considering an isolation TX.

Be aware that if ever a genset is used there is no  NE bond on many smaller generator designs, and to use conventional ADS methods (RCDS and breakers) an NE bond needs to be added, usually  inside  the 1st plug. However it then needs to be not there when 'plugged in' to a building, or an RCD in that building will see an NE connection as a fault and trip....

Mike   

I was picturing in my head a horse lorry with living accommodation or a large motor home built on a lorry chassis cab.

So, assuming the lorry has a 24 volt battery and electrical system, which is 28.8 volts when the engine and alternator is running and a 12 volt leisure battery  which is 14.4 volts when charging from the lorry electrical system, allowing the use of 12 volt electrical equipment built for motor homes and other vehicles, as well as 24 volt equipment built for lorries, some of which have cabs fitted out as living accommodation. So in my head I was picturing the DC return paths to the battery simply being through the rack and the bolts securing the rack to the metal vehicle body and chassis, with the equipment grounded to the rack and with bonding only being needed to complete the circuit if rack is secured to a fibreglass body or similar.

If you are planning to run this equipment from the mains directly by reducing the voltage and converting to DC rather than just running of the vehicle electrics and it's a whole different ball game. 

It was probably around six years ago I posted on this forum and had a conversation with the technical team at a shower waste pump manufacturer asking why their pumps and controls needed RCD protection, when the pump is actually a DC 24 V (28.8 volt) gulper bilge pump originally designed and built for pumping water out of boats. 

It was hard to get the technical team to say it,  but there's a lack of full electrical separation between the 24 and 230 in their equipment,  so you could get a AC 230 volt shock off the DC 24 volt circuit and pump.

So, I am not going to give you any advice  because what you require is dependent upon what equipment you use and how you connect it. 

You need to use good quality equipment, follow the manufacturers instructions and ask them for any additional information you require. 

Sparkingchip said:

and with bonding only being needed to complete the circuit if rack is secured to a fibreglass body or similar.

Surely that would not be bonding.