Isolation monitoring on non-earthed mains systems

[Diver300]

I'll be working on a mains generator which uses the IT (non-earthed) system. There is an insulation monitoring system which measures the resistance between the mains supply and earth, and shut off if there is conduction.

I had not come across this sort of system until recently. Does anyone has any experience of them, and how well they work in practice?

The system uses an imposed DC voltage to measure the resistance, and I wanted to know how it will react to real-life faults, not just ideal resistors between live or neutral and earth.

 

[Tony Stewart]

As you know spectral leakage current to ground is a more common fault for EMI.

I would think an AC conduction method is better just as it is for ground moisture.

 

[Externet]

Are minor capacitive coupling leaks to be considered ? ( Live conductors-to-grounded conduit.)
Are capacitive leaks to consider between live phases ? Guessing the IT generator is AC ?

A current transformer with a very high turn count comes to mind...

 

[Diver300]

Are minor capacitive coupling leaks to be considered ? ( Live conductors-to-grounded conduit.)
Are capacitive leaks to consider between live phases ? Guessing the IT generator is AC ?

A current transformer with a very high turn count comes to mind...

The IT generator is AC, same voltage and frequency as domestic mains.

Capacitive leaks between live phases would be indistinguishable from a capacitive load, such as a power factor capacitor or a transformerless power supply, so those are not considered, but they wouldn't be with an earth leakage breaker on a grounded system.

On the system that I'll be working on, capacitive leaks, like the Y-capacitors in some appliances, are not to be considered.

 

[aliarifat794]

IMS systems are tuned to minimize false alarms due to normal operational conditions, like minor capacitive leakages or transient conditions. However, they can be sensitive to changes in insulation quality, so they might trip if there’s a sudden or substantial degradation in insulation.

 

[picbits]

Yup, use these extensively at work on both 1000-1200v DC systems and three phase IT systems.

Our go to is the Bender iso685 units as we deal a lot with the higher voltages. They do the job but you need to be wary of a few gotchas ...

Pure DC with total isolation is one application we use them for, if there is any damage to the screened cables we use (earthed) then it will pick this up and has on more than one occasion identified a problem. They are also used to determine if the DUT has any leakage which can happen when you get breakdown in the internal insulation due to heat or moisture ingress etc. Filtering on the DC outputs and AC converters tends to start bringing the total value down.

Using them for 3ph AC on a transformer isolated system (500kW+) also works well but ......

Start plugging in mains to neutral 240v devices or 3ph power supplies with internal to earth filtering and you will soon start bringing down the total insulation resistance. A couple of weeks ago we were watching the resistance go from 300K+ down to around 30K as we plugged in every day devices with filters in.

Reaction times are in the seconds once a fault is detected, you can watch the resistance values ramp down every second until the alarm event happens. Sometimes 3-4 seconds. Not an RCD reaction but good for continual monitoring as these will let you know about insulation breakdown before you even turn on the power.

 

[picbits]

Worth a read though the application sheets here : https://www.ti.com/tool/TIDA-010232

Above is for DC but it gives you some good insight on how they work.

 

[Diver300]

Start plugging in mains to neutral 240v devices or 3ph power supplies with internal to earth filtering and you will soon start bringing down the total insulation resistance. A couple of weeks ago we were watching the resistance go from 300K+ down to around 30K as we plugged in every day devices with filters in.

As I understand it, mains filters contain capacitors. Are you saying that the isolation monitoring is sensitive to capacitors?

 

[Nigel Goodwin]

As I understand it, mains filters contain capacitors. Are you saying that the isolation monitoring is sensitive to capacitors?

Obviously, capacitors pass AC current - it's also a major cause of (incorrectly tested) PAT inspections.

 

[Diver300]

Obviously, capacitors pass AC current - it's also a major cause of (incorrectly tested) PAT inspections.

The isolation monitoring system that I have been looking at uses DC to monitor the isolation resistance. As such, it will not be detecting AC currents at all, so filter capacitors will not be seen as isolation faults. That was why I was asking.

I'm not sure if other isolation monitoring systems ignore capacitive connections.

 

[picbits]

We've found older supplies have some leakage through the capacitors which can be picked up as insulation resistance (as well as probably some other paths through to earth through other components (voltage dividers for monitoring and badly wired stuff). Also seen issues with heater and heating elements changing insulation resistance as they get hot so if you see this happening, worth bearing in mind.