Question

When a battery is disconnected, the earth lead should always be disconnected first because

Answer 1

Well obviously, you dont want the engine to turn over while your working on it. So disconnecting the battery is crucial. I recommend disconnecting both terminals. But if only one terminal is disconnected, then it should be the negative terminal. There is only one reason for this. Because the negative end of the battery is connected to ground. It is only a slight advantage to disconnecting the positive terminal. The reason for this is because while connected, any part of the chassis will complete the circuit and the chassis is one big wire that is easy to bump into.
hope this helps you mate <3

Answer 2

But if only one terminal isdisconnected, then it should be the negative terminal. There is only one reason for this. Because the negative end of the battery is connected toground.