Last edit: 07/09/2023
Bonding is one of the most discussed topics in Machinery. There are several reasons to bond a conductive metal part; one of those reason is to protect control circuits against malfunctioning.
The prescription is in both IEC 60204-1 and NFPA 79.
[IEC 60204-1] 9.4.3 Protection against malfunction of control circuits. 188.8.131.52 Insulation faults
184.108.40.206.2 Method a) – Earthed control circuits fed by transformers. The common conductor shall be connected to the protective bonding circuit at the point of supply. All contacts, solid state elements, etc., which are intended to operate an electromagnetic or other device (for example, a relay, indicator light) are to be inserted between the switched conductor of the control circuit supply and one terminal of the coil or device. The other terminal of the coil or device is connected directly to the common conductor of the control circuit supply without any switching elements.
[NFPA 79] 8.3 Control Circuits.
Control circuits shall be permitted to be grounded or ungrounded. […]
8.3.1 If the control system is grounded, the output shall be grounded as near as practicable to the control power source and before the first control device. Switching devices shall not be permitted in a grounded conductor(s) unless the control circuit conductor(s) is opened simultaneously.
[NFPA 79] 9.1.4 Connection of Control Circuit Devices.
220.127.116.11 All operating coils of electromechanical magnetic devices and indicator lamps (or transformer primary windings for indicator lamps) shall be directly connected to the same side of the control circuit. All control circuit contacts shall be connected between the coil and the other side of the control circuit.
Two possible consecutive faults are prevented with the above language, both linked to a ground fault in the control circuit.
Wrong connection of the Emergency Stop button
In figure 1, the control circuit electrical contact, an emergency push button in this case, is connected between the contactor coil and ground: that is not allowed! All control circuit electrical contacts must be connected as indicated in figure 3.
The reason is that, in case the emergency pushbutton is connected like in figure 1, in case of a ground fault, as indicated with the red arrow, its function is bypassed, or in other terms, the emergency pushbutton becomes inactive!
That is considered a systematic failure. Another way to say the same concept is that it is a violation of a Basic Safety Principle stated in ISO 13849-1:2012 Table D.1. It means that with that wrong wiring it does not make any sense to calculate the reliability level of the safety-control circuits (SRP/CS or SCS).
Situation in case of two faults: again a wrong connection!
In case the same emergency push button is connected like indicated figure 2, in case of two faults in the field, as indicated by the arrows, the emergency pushbutton is bypassed again!
Correct wiring and bonding in a control circuit
In order to avoid possible systematic failures in control circuits, it is important to ground (bond) one of the two poles of the transformer and all metal parts that contain control circuits, even in case of Extra-low voltage sources. By doing that, when the first ground fault happens in the field, like shown in figure 3; the overcurrent protection, indicated as “I >”, de-energises the circuit and places the machinery into a safe state.