When designing a complex system that incorporates different grounding types (TN/TT and IT), we must consider the safety characteristics of each. How can we cleverly utilize earth leakage relays and insulation monitoring devices to improve safety?
When designing a complex system that incorporates different grounding types (TN/TT and IT), we must consider the safety characteristics of each. How can we cleverly utilize earth leakage relays and insulation monitoring devices to improve safety?
I would add that, whilst it is permitted by the standards to use IMD and RCD, along with other protective devices, there are some important notes, particularly around the use of IMDs and RCMs:
1. An IMD is not a "protective device". It can be used to monitor for a "first fault" in IT systems, and initiate disconnection (or action by someone to remove power or provide ADS (for second fault in IT system, or faults to Earth in TN and TT systems).
2. An RCM, similarly, is not a "protective device. It also can be used to monitor for a "first fault" in IT systems, and initiate disconnection (or action by someone to remove power or provide ADS (for second fault in IT system, or faults to Earth in TN and TT systems).
3. An RCD or OCPD can be used as a protective device to achieve ADS for a second fault in IT system, or for faults to Earth in TN and TT systems.
The reason for statements in 1. and 2., is that there are no standards to ensure disconnection for relays, contactors or other disconnectors actuated by these monitoring devices ... they are, as stated, 'monitoring' devices.
In TN and TT systems, faults to Earth lead to an immediate shock risk, so use of RCM or IMD without guaranteed disconnection time is not really a safety improvement.
In IT systems, yes there is an improvement in safety to provide some action, including causing supply disconnection for the affected circuit, after first fault (for which there need not be a danger ... BS 7671 and IEC 60364 don't really account for multiple faults).
Finally, as to whether RCMs can be used with IMDs, that might depend how they work, i.e. whether they are compatible.
I would add that, whilst it is permitted by the standards to use IMD and RCD, along with other protective devices, there are some important notes, particularly around the use of IMDs and RCMs:
1. An IMD is not a "protective device". It can be used to monitor for a "first fault" in IT systems, and initiate disconnection (or action by someone to remove power or provide ADS (for second fault in IT system, or faults to Earth in TN and TT systems).
2. An RCM, similarly, is not a "protective device. It also can be used to monitor for a "first fault" in IT systems, and initiate disconnection (or action by someone to remove power or provide ADS (for second fault in IT system, or faults to Earth in TN and TT systems).
3. An RCD or OCPD can be used as a protective device to achieve ADS for a second fault in IT system, or for faults to Earth in TN and TT systems.
The reason for statements in 1. and 2., is that there are no standards to ensure disconnection for relays, contactors or other disconnectors actuated by these monitoring devices ... they are, as stated, 'monitoring' devices.
In TN and TT systems, faults to Earth lead to an immediate shock risk, so use of RCM or IMD without guaranteed disconnection time is not really a safety improvement.
In IT systems, yes there is an improvement in safety to provide some action, including causing supply disconnection for the affected circuit, after first fault (for which there need not be a danger ... BS 7671 and IEC 60364 don't really account for multiple faults).
Finally, as to whether RCMs can be used with IMDs, that might depend how they work, i.e. whether they are compatible.
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