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?
IT systems with insulation monitoring devices are ubiquitous on ships and very common offshore. However, there is conflicting guidance from HSE and IMD manufacturers about the applicability of earth leakage protective devices in IT systems.
Guidance from HSE basically recommends use of RCDs on final circuits and implies that these may indeed tip on the 1st fault:
"The IT electrical distribution system insulation impedance to earth will often be at a value low enough to allow an RCD to function when an earth fault occurs. This is particularly so with electrical distribution systems that incorporate harmonic line filters."
It also makes the point that the fault impedance of any 1st fault may be high enough the limit the fault current of any 2nd fault such that the protective device doesn't operate correctly.
https://www.hse.gov.uk/offshore/infosheets/is2-2011.htm
However, a pamphlet from the IMD manufacturer Bender argues that RCDs should only be used in IT systems in exceptional cases only. This is because depending on the position of the RCD in the circuit, any leakage current via parasitic capacitance in the supply cable, providing the return path, may appear behind (load side of) the RCD and prevent it from "seeing" any differential current. For this reason the RCD, if installed, is recommended to be located as close to the load as possible. In situations where you have parallel RCDs it difficult to guarantee that the device actually associated with the circuit under fault will be the one that trips.
If IT systems are incorporated with IMDs it's imperative that operators are trained to find and rectify 1st fault situations without delay.
For systems where IT is selected for fault tolerance of critical loads, one option is to have TN systems with 30mA RCDs, downstream of the IT network, on parts of the system that are less critical and more likely to have earth faults e.g. socket outlets, laundry and galley equipment etc
IT systems with insulation monitoring devices are ubiquitous on ships and very common offshore. However, there is conflicting guidance from HSE and IMD manufacturers about the applicability of earth leakage protective devices in IT systems.
Guidance from HSE basically recommends use of RCDs on final circuits and implies that these may indeed tip on the 1st fault:
"The IT electrical distribution system insulation impedance to earth will often be at a value low enough to allow an RCD to function when an earth fault occurs. This is particularly so with electrical distribution systems that incorporate harmonic line filters."
It also makes the point that the fault impedance of any 1st fault may be high enough the limit the fault current of any 2nd fault such that the protective device doesn't operate correctly.
https://www.hse.gov.uk/offshore/infosheets/is2-2011.htm
However, a pamphlet from the IMD manufacturer Bender argues that RCDs should only be used in IT systems in exceptional cases only. This is because depending on the position of the RCD in the circuit, any leakage current via parasitic capacitance in the supply cable, providing the return path, may appear behind (load side of) the RCD and prevent it from "seeing" any differential current. For this reason the RCD, if installed, is recommended to be located as close to the load as possible. In situations where you have parallel RCDs it difficult to guarantee that the device actually associated with the circuit under fault will be the one that trips.
If IT systems are incorporated with IMDs it's imperative that operators are trained to find and rectify 1st fault situations without delay.
For systems where IT is selected for fault tolerance of critical loads, one option is to have TN systems with 30mA RCDs, downstream of the IT network, on parts of the system that are less critical and more likely to have earth faults e.g. socket outlets, laundry and galley equipment etc
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