Electrical bonding systems in portable generators


An electrical bonding system is crucial in a portable generator, and an RCD trip will warn of faults at the appliance end. Gary Busbridge explains.

Electricity may be governed by the fixed laws of physics, but electrical Standards are constantly being reviewed and updated.

Fortunately, it is now the turn of the generator Standard. And many of you will think it’s about time, as there have been several concerns about generator installations.

This article deals with portable gen-sets. These lightweight units are single phase, have a socket outlet, are double insulated and don’t require an earth stake (nor should they). The plug and socket connection is usually temporary, and because power tools are also double insulated there is inherent safety protection.

In Australia gen-sets are mainly used for camping trips or on the back of a ute to power tools on a work site.

These applications are also prevalent in New Zealand, but there’s a third that is quite understandable. Home owners use portable gen-sets to power their homes in the case of emergency – and they have endured multiple natural disasters.

However, a word of caution is required. There are many centre-tapped gen-sets in the world, and they are inherently dangerous when plugged into a home. The centre tap on the windings gives rise to a lot of problems, as only one side of the windings is protected.

Serious fires and electric shocks have been experienced in New Zealand.

The recommended – and safest – gen-set is the single-phase type, which effectively has an active and neutral connection from the windings. A socket outlet is provided so that an extension cord can provide power to an appliance. (The socket outlet must have a double-pole switch to ensure that active and neutral are switched.)

The extension cord may be connected to an appliance inlet on an installation, but the installation must have a changeover switch so there is no chance of the gen-set feeding into the grid or exposing live parts. Any devices set up to accept a gen-set must be marked to indicate their purpose.

Most of you would have seen the image of a gen-set with an earth electrode in a bucket of dirt. It’s a bit of a laugh, but I hope none of you are guilty of such antics.

It is imperative to ensure bonding is carried out. In a normal gen-set all parts should be electrically bonded to form a gen-set bonding system.

The parts should include the engine frame, the generator frame, all exposed conductive parts, the earth terminals of the socket outlet and the frame connection. This will ensure that all parts are electrically connected in a secure manner with appropriate low impedance throughout.

If you have a gen-set with a residual current device (RCD) fitted, then these should be electrically connected between the generator winding and the frame.

The gen-set bonding system mentioned should be connected at the appropriate point to one side of the gen-set windings – and there should be only one such connection.

The input neutral terminal of the RCD should be connected to the earth terminal of the socket outlet and to the generator frame, subsequently providing an MEN connection. In most instances this is a combined socket outlet and RCD in the one device, meaning that the neutral to earth connection is across the back of the terminals on the socket outlet.

To ensure the functionality of the RCD socket outlet, the appropriate tester should be used to check that at 30mA the unit trips out in the required time.

Check that the neutral terminal of the gen-set upstream of the RCD is connected to the frame terminal, and that the earth terminal of the socket outlet is connected to the gen-set bonding system.

Ensure that the neutral wire downstream from the RCD is not electrically connected to the gen-set bonding system.

Always remember that a gen-set bonding system is not required to be connected to the general mass of earth through an earth electrode. The following circumstances do not allow for an earth electrode:

  • the gen-set supplies 230V power via an approved three-pin flat-pin plug and socket arrangement;
  • the gen-set is mounted on or sitting on a truck or tray-top, where the gen-set frame is in direct contact with the metal frame of the surface on which it is sitting; and
  • the gen-set, when in use, is sitting on the ground soil, concrete floor or slab, or a road.

Note that if the gen-set cannot be in contact with a mass of earth or with a conductive path between the frames and a surface, then a safe electrical configuration must be determined before use.

The current that may flow in the case of a fault should be kept as low as possible by ensuring that an earth electrode is not fitted. The greater resistance created by surface contact limits the current flow, which in turn increases the earth leakage protection offered by the RCD.

An RCD trip will provide a warning that a faulty appliance is connected to the socket of the gen-set. If there is a fault from a live conductor touching an appliance casing, thereby creating a path to earth in the gen-set, it will cause an imbalance between the active and neutral in the RCD.

Ultimately this will trip the RCD. However, without an RCD on the gen-set, a faulty appliance could continue to work even though inherently unsafe.

Although the RCD will trip if there is an imbalance of active and neutral, it is highly unlikely that the current would pass through the operator to earth, as there is no path for the fault to follow.

Another recommendation is to conduct the usual RCD check as per the AS/NZS 3000 Wiring Rules – that is, press the ‘test’ button before each use of the gen-set, and periodically verify the trip current and trip time with testing devices at your workshop.

So use these gen-sets as they are intended and you will have no problems. On that camping trip the beer will always be cold and the meat always fresh.

About Gary Busbridge

Gary Busbridge

Gary Busbridge has been with Clipsal for more than 33 years. Since 1997, he has been involved with Standards Australia and has held memberships in several Standards committees.

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