Double Bonding

Sometimes, we notice electric motors having a Double Bonding. It is recognisable because there is a single PE conductor connected with the motor frame. The second PE cable is normally not visible since it is inside the cable connected inside the motor terminal box. Bottom line: the motor is bonded twice:

• First time inside the terminal box
• Second time on the motor frame

Is this double bonding mandatory?

The starting point is the mother of all electrical Safety Standards: IEC 60364-4-41. Inside that standard, criteria are provided for fault protection. Fault Protection covers protection under fault conditions: it is the new terminology to indicate the protection against indirect contact.

[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)

Automatic Disconnection of supply is, by far, the most used method to protect against indirect contacts. That requires:

• The bonding of all exposed conductive parts
• A protective device that automatically switches off the supply, within a well defined disconnection time.

The bonding of a motor frame is achieved using a single PE cable connected to the grounding terminal, located inside the motor terminal box. There is no need to connect a second cable to the frame!

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 bonding 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.

Despite some motors come with two terminals for bonding the metal frame, for fault protection, it is enough to connect the terminal inside the terminal box to a PE cable. There is no Normative requirement to connect the “frame PE terminal” to a bonding cable (or equipment Grounding Conductor, to use a North American Language).

Therefore, in case the motor only comes with the provision to bond it inside its terminal box, there is no normative requirement to do a second bonding on the motor frame, like in the picture at the side.