Double Bonding

Sometimes, we observe electric motors equipped with double bonding for protective earthing (PE). This is often noticeable because a separate, clearly visible PE conductor is connected directly to the motor frame. In addition to this, there is usually another PE conductor integrated within the supply cable (alongside the three phase conductors), which is connected inside the motor terminal box.

In summary, the motor has two bonding points:

1. Internal bonding – via the PE conductor within the terminal box (from the power cable)

2. External bonding – via an additional, visible PE conductor connected to the frame

Question

Is this double bonding mandatory according to standards (e.g. IEC 60364, IEC 60204-1), or is it just an additional safety measure?

The starting point is the foundation of all low-voltage electrical safety standards: IEC 60364-4-41. This standard sets the fundamental requirements for fault protection, which is the modern terminology replacing “protection against indirect contact.”

Fault protection refers to protective measures implemented to prevent electric shock in the event of an insulation fault, where exposed-conductive-parts (such as the metal frame of an electric motor) may become live. IEC 60364-4-41 defines how these parts must be connected to the protective earthing system to ensure automatic disconnection of supply under fault conditions.

[IEC 60364-4-41: 2005] 410.3.4 Generally, the following protective measures may be applied:

– automatic disconnection of supply (Clause 411),

– double or reinforced insulation (Clause 412),

– electrical separation for the supply of one item of current-using equipment (Clause 413),

– SELV systems (Clause 414),

– PELV systems (Clause 414)

The most widely used method for protection against indirect contact is Automatic Disconnection of Supply (ADS). This method requires two fundamental conditions:

• Bonding of all exposed-conductive-parts

• A protective device (such as an MCB or fuse) capable of disconnecting the supply within a defined disconnection time in case of a fault

For electric motors, the bonding of the frame is normally achieved through a single protective earthing (PE) conductor connected to the earthing terminal inside the motor terminal box. This ensures that any fault current will flow back to the source through the PE conductor, enabling automatic disconnection.

There is no requirement in IEC 60364-4-41 to add a second external PE conductor to the frame. The internal PE connection is sufficient to meet the fault protection requirements—as long as continuity and proper conductor sizing are ensured.

But why, then, some motors come with a second terminal? That is because in the IEC 60034-1: Rotating electrical machines – Part 1: Rating and performance, the following language is present:

[IEC 60034-1: 2022] 11.1 Protective earthing of machines

[…] In the case of machines having rated voltages greater than AC 50 V or DC 120 V, but not exceeding AC 1 000 V or DC 1 500 V, the terminal for the earthing conductor shall be situated in the vicinity of the terminals for the line conductors, being placed in the terminal box, if one is provided. Machines having rated outputs in excess of 100 kW (or kVA) shall have in addition an earthing terminal fitted on the frame.

The reason is to make it easier to connect a PE cable, or, if the user wants, to create a second fault loop. However, neither IEC 60364-4-41 nor IEC 60204-1 require that second fault loop.

Although some motors are equipped with two PE terminals—one inside the terminal box and another directly on the frame—for fault protection purposes, it is sufficient to connect the internal PE terminal (inside the terminal box) to a protective earthing (PE) conductor.

There is no normative requirement in IEC 60364 series to additionally connect the external frame terminal with a second bonding conductor (often referred to as an equipment grounding conductor in North American terminology).

Therefore, if a motor is supplied only with an internal earthing provision inside its terminal box, this single connection fully satisfies the requirements for fault protection. A second PE conductor applied directly to the motor frame—such as the one shown in the picture—is not mandated by any standards, but rather an optional measure, sometimes added for redundancy.