Earthing DC System

It depends on the situation, you need to give more information. My holistic hat is away for recalibration at the moment.

if you are using NEC code then,

Per NEC Section 250.162(A) requires grounding for two wire DC systems that are greater 50 volts but not greater than 300 volts, so most 250 volt DC systems must be grounded.

DC system up to 50 volts ( most modern fire alarm circuit and most DC instrumentation, communication and data circuits ) are not required to be grounded and NEC has no rule over 300 volts too.

another point is that all three wire DC systems are required to be grounded regardless of voltage.

what is a 3 wire dc system? what are the potential difference between each conductor? ie...

phase +48v

Neutral 0v

ground 0v

A three wire DC system is just that. Take two DC sources (for argument sakes - two batteries), connect them in series so that the negative of one source is connected to the positive of of the other one.

Voltage across the series pair will be the sum of both sources. Now pull a wire out from the junction point between the two sources and call it a "neutral". Ground that neutral conductor.

Potential of Source A to the neutral will be the same as the potential of Source B to the neutral accept that the polarity is reversed.

It is also common to do this with the sources connected negative to negative. In this case, there is no potential across the series pair and there is no difference in polarity between one source to the neutral versus the other one to the neutral.

Without an input from ronald A we can hypothesise for ever and be no where near the required answer.

the system is for the Engine battery which is 24vdc, should we earth the negative side of the battery.

Now why didn't you say that in the first place? The answer is yes. It must be a substantial bonding connection as it will be carrying the load of the starter motor.

You do have one 24v battery and not two 12v ones in series right?

If you have just 1 battery, ground the negative. If you have two batteries in series, do not ground the negative of the battery that connects to the positive of the second battery, but still ground the negative on the second battery.