Last edit: 07/02/2026
THE DOUBT:
In the design of the electrical system, how can I distinguish which metal parts are exposed conductive part? Is bonding all metal parts a valid safety measure, or can it even be counterproductive?
CONSIDERATIONS:
The difference between Exposed Conductive Parts (ECP) and a normal conductive part can be un unclear. For an exact definition of exposed conductive part we refer to IEC 61141:
[IEC 61141-3.6] Exposed-conductive-part: conductive part of equipment, which can be touched, and which is not normally live, but which can become live when basic insulation fails.
How should metal frames in direct contact with an exposed conductive part be considered? Since they can become energised when the exposed conductive part itself is subject to a ground fault and becomes energise? Again, the standard clarifies the situation:
[IEC 61141-3.6- Note 1] ” A conductive part of electrical equipment which can become alive only through contact with an exposed conductive part which has become live, is not considered to be an exposed conductive part itself.”
For example, the frame of an electrical motor is an Exposed conductive part, while his metal basement is not an exposed conductive part, although, in the event of a ground fault, both can become energised.
Every exposed conductive part, in order to be adequately protected for indirect contact (Fault protection), have to be bonded. However, this fundamental rule is often misinterpreted; here are, for example, some incorrect statements regarding exposed conductive part and grounding:
- Anything that can become energised must be bonded.
- Metal parts that do not move must be grounded (if it doesn’t move, than ground it).
- Grounding “discharges” the fault current to the ground.
- Grounding keeps the exposed conductive part at ground potential (zero), even in the event of a failure…. Very wrong!!!
Earthing (grounding) is intended to create a low-impedance fault loop that allows sufficient fault current to flow so that protective devices—such as circuit breakers or RCDs—can detect the fault and disconnect the supply.
It does not discharge current into the earth, nor does it force exposed conductive parts to assume earth potential. If that were the case, touching an exposed conductive part during a ground fault would present no risk of electric shock, and overcurrent or residual current protection would be unnecessary.
Misunderstanding this principle often leads to the incorrect practice of bonding all large metal parts, including those that are not exposed conductive parts and are not liable to become live in the event of a fault. This practice is not only unnecessary but can also be dangerous.
Only exposed conductive parts—that is, conductive parts that can become live under fault conditions—shall be connected to the protective earth (PE) conductor. Let’s, for example, look at the two scenarios in the figure below.
The example is developed in the context of the U.S. market (480 V AC phase-to-phase), but the same principles apply equally in Europe. In both cases, the motor is mounted on anti-vibration pads, so the motor is not in direct contact with the dark grey conductive part. Two persons are involved: Person A, who touches the motor frame, and Person B, who touches a separate conductive part that is in mechanical contact with the motor frame.
Situation A – Conductive part not connected to PE
The conductive part touched by Person B is not connected to a protective earth (PE) conductor.
In the event of an insulation fault in the motor stator winding, the motor frame becomes energised. Person A is therefore exposed to an electric shock, while Person B is not.
In this scenario, severe injury for person A is unlikely because the overcurrent protection device (OCPD) will clear the fault by opening the fault loop within the prescribed disconnection time (typically 0.2 s in the U.S. and 0.4 s in Europe).
Situation B – Conductive part connected to PE
The conductive part touched by Person B is connected to the protective earth (PE) conductor.
In the event of the same insulation fault, both the motor frame and the bonded conductive part rise to a dangerous touch voltage until the OCPD disconnects the supply. As a result, both Person A and Person B are exposed to an electric shock.
Again, fatal injury is unlikely because the OCPD will disconnect the fault within the required safety time.
Safety assessment
The key question is: which situation is safer?
Situation B is clearly worse. By connecting a PE conductor to a conductive part that is not an exposed conductive part, the fault is unnecessarily extended to additional accessible metalwork, increasing the number of persons exposed to hazardous touch voltages.
This example illustrates why only exposed conductive parts—that is, a conductive parts that can become live under fault conditions—shall be connected to the protective earth. Bonding conductive parts that are not exposed conductive parts does not improve safety and can, in fact, increase the overall risk.
Similar situations frequently arise with fixed shelters, metal doors, frames, and structural supports. Connecting these parts to a protective earth (PE) conductor can increase the risk in the event of an electrical fault rather than reduce it.
For example, a metal door of an electrical room—such as the red door of a panel room—cannot normally become live, as it is sufficiently distant from any electrical equipment and not subject to insulation failure. If this door is nevertheless connected to a PE conductor, then during an earth fault occurring anywhere in the installation, the door may be brought to a hazardous touch voltage. In this case, the PE connection does not improve safety; instead, it reduces the overall safety level of the plant by unnecessarily extending the fault to accessible metalwork.
CONCLUSIONS
Among the conductive parts of an electrical installation, only those that may become energised as a result of a failure of the main insulation shall be considered exposed conductive parts.
Once identified, all exposed conductive parts must be connected to the protective earth (PE) conductor in order to ensure automatic disconnection of supply and to prevent the risk of electric shock.
Conversely, conductive parts that are not exposed conductive parts shall not be connected to the PE conductor. Bonding such parts does not provide additional protection and may increase the risk to persons by spreading dangerous touch voltages during an electrical fault.