Can earth leakage relays and insulation monitoring be used together?

Although casualy it may seem that the 2 devices perform a similar fuction - to warn and then remove the supply if there is a leakage path from a live conductor to earth, there are  in effect solutions for 2 very different situations here. 

Normally insulation monitoring is associated with floating (IT) systems where there is almost no CPC current by design, so leakages to earth of a scale that would be early  warning of insulation failure can be detected. Typically thresholds corresponding to insulation resistances of hundred plus k  ohms, and only a small circuit and a few dedicated devices would be protected by a single detector.  Also it can be arranged, as initial leakage is not dangerous, to alert but not to disconnect, so the current process may be safely concluded before shutting down for repair. This is an expensive approach, but may be justified in some settings, perhaps, say in a surgical theatre.

Earth fault relays are in effect RCDs with more options, and are looking at the imbalance between live and neutral conductors - the assumtion being that whatever goes out but does not come back, has probably done something bad, and worthy of operating the Automatic Disconnection. This is normally done with one relay covering a TN-x system or reasonable extent, so there will be CPC currents as a matter of course, due to wiring capacitance and the nature of the likley loads, that amount to a few tens of mA. The most sensitive RCDs in common use have a threshold of 10mA, and 30mA devices are the CENELEC standard for safety of life.

However, a 10mA CPC current let alone a 30mA one, is a not really an early warning of insulation failure,  rather at 25k ohms for 10mA and ~ 7k for 30mA, it is only detecting pretty much total failure sometime after the fact. If that level of fault occurs at a single point, (in a volume of a few cubic centimetres perhaps ) it is likely to be noticeably cooking, and there is a risk of ignition. As such the normal sensible option is immediate ADS rather than pre-warning.

I'm  not seeing any benefit from combining the two, as one more or less excludes the other.

Mike.

Although casualy it may seem that the 2 devices perform a similar fuction - to warn and then remove the supply if there is a leakage path from a live conductor to earth, there are  in effect solutions for 2 very different situations here. 

Normally insulation monitoring is associated with floating (IT) systems where there is almost no CPC current by design, so leakages to earth of a scale that would be early  warning of insulation failure can be detected. Typically thresholds corresponding to insulation resistances of hundred plus k  ohms, and only a small circuit and a few dedicated devices would be protected by a single detector.  Also it can be arranged, as initial leakage is not dangerous, to alert but not to disconnect, so the current process may be safely concluded before shutting down for repair. This is an expensive approach, but may be justified in some settings, perhaps, say in a surgical theatre.

Earth fault relays are in effect RCDs with more options, and are looking at the imbalance between live and neutral conductors - the assumtion being that whatever goes out but does not come back, has probably done something bad, and worthy of operating the Automatic Disconnection. This is normally done with one relay covering a TN-x system or reasonable extent, so there will be CPC currents as a matter of course, due to wiring capacitance and the nature of the likley loads, that amount to a few tens of mA. The most sensitive RCDs in common use have a threshold of 10mA, and 30mA devices are the CENELEC standard for safety of life.

However, a 10mA CPC current let alone a 30mA one, is a not really an early warning of insulation failure,  rather at 25k ohms for 10mA and ~ 7k for 30mA, it is only detecting pretty much total failure sometime after the fact. If that level of fault occurs at a single point, (in a volume of a few cubic centimetres perhaps ) it is likely to be noticeably cooking, and there is a risk of ignition. As such the normal sensible option is immediate ADS rather than pre-warning.

I'm  not seeing any benefit from combining the two, as one more or less excludes the other.

Mike.