Why install ground first?
Most products used in auto-electrical systems can be considered two terminal devices, meaning there are only two connections (power and ground). These products include:
- Batteries
- Starters
- Alternators
- Electric motors
- Lamps
For these two terminal devices, it does not matter to the internals of the device [AM1] in which order the connection is completed. Other factors like safety, convention or habit will determine order of connection.
Many other products in vehicles, however, should be considered three terminal devices. They have at least one input, at least one output and a common ground terminal.
Examples of three terminal products include:
- Most voltage converters
- Electronic ignition systems
- Controllers (for example Ignition Headlight Controllers)
- Timers (for example Turbo Timers, Glo-plug Timers, Oiler Timers, Courtesy Light Timers)
- Voltage sensors (for example Crank Disable Sensors, Load Disconnect Sensors)
The ground terminal provides a reference point and a path for drive current for internal components.
All three terminal devices, not just common ground voltage converters, should have ground connected first and disconnected last. They should never be switched on and off via a switch in the earth line unless explicitly recommended by the manufacturer.
For example, some voltage converters, if they have input and output connected before ground is connected, will start to operate without this reference. This means there is no internal reference to zero volts and no sink for drive current. This may cause erratic modes of operation such as:
Input voltage appearing on output with no ground to allow over-voltage protection to operate. This may lead to the damage of equipment on output.
No start up drive for internal components, meaning the converter may not start when earth is eventually connected. This may lead the installer to mistakenly believing the unit is faulty.
There are many other scenarios involving other types of three terminal products where similar or worse problems can arise if ground is connected last or disconnected first.
The same thing applies with switching on/off. An on-off switch must never be placed in the earth line of a three terminal system (unless it is specifically recommended by the manufacturer).
Some manufacturers design products to avoid, as much as possible, problems associated with loss of earth but never assume it is a safe practice to have power on any three terminal product without earth.