Earthing and bonding, and the difference between them

The two words are used interchangeably and describe different mechanisms, which matters because a system can be well earthed and badly bonded, and the failure that results is a shock hazard.

Bonding

Connecting conductive parts to each other so they are at the same potential.

A person touching two bonded surfaces experiences no voltage between them, because they are joined. That is what makes a machine frame safe to touch, and it does not depend on a connection to earth at all.

Bonding is why a bird on a power line is unharmed: both feet are on the same conductor, so there is no potential difference across the bird.

Earthing

Connecting the bonded system to the general mass of earth.

The purpose is fault current. A live conductor touching an earthed frame must produce enough current to operate the protective device quickly. Without a low impedance path back to the source, that current is small, the fuse does not blow, and the frame stays live.

The number that matters is not the resistance to the soil. It is the impedance of the fault loop back to the supply transformer, because that is what determines the fault current and therefore the disconnection time.

Functional earth

The third thing, and the one that causes confusion in control panels.

A functional earth is a reference for a circuit to work: the zero volt reference of an analogue system, the return for a cable screen, the reference for an instrument.

It is not a protective conductor and must not be relied on as one. It should also not share a conductor with fault current, because fault current flowing through a signal reference is a very large transient on that reference.

In practice: separate functional earth bar, bonded to the protective earth at exactly one point, which is the star point.

The star point

One point where everything comes together, and the arrangement that prevents the commonest earthing fault in a panel.

Daisy chaining earths, device to device to bar, means every device's return current flows through the earth conductor of every device upstream of it. Each conductor has a small resistance, so each device sits at a slightly different potential, and the difference varies with load.

For power that is a nuisance. For an analogue reference it is a measurement error that changes when a contactor pulls in.

Each device gets its own conductor back to the bar. The bar is bonded to the incoming protective earth at one point.

Doors, plates and hinges

The most common inspection finding.

A hinge is a mechanical bearing. Its electrical resistance depends on paint, grease, wear and how hard the door was last shut, and it is not a protective conductor.

Any door carrying equipment needs a bonding strap, flexible, properly terminated at both ends. Mounting plates need a bonding conductor rather than relying on the paint being scraped by a bolt.

Measuring it

A continuity test with a multimeter proves there is a connection and proves very little else.

A proper earth continuity test uses a substantial current, tens of amps, because a connection that passes 200 milliamps can fail at fault current levels. A joint with a resistive film reads fine on a multimeter and glows under fault.

The test that matters for disconnection time is an earth fault loop impedance test at the furthest point, compared with the maximum permitted for the protective device fitted.

The three failures worth checking first

• A screen terminated as a pigtail, discussed elsewhere and belonging here too.
• A functional earth and a protective earth bonded at more than one point, creating a loop that carries current.
• A panel earthed to a machine frame that is itself earthed elsewhere, so the two earths are at different potentials and the difference appears across every signal between them.