MFT for a DIYer?

Pete999 · Feb 6, 2021

timhoward said:
Any idea why this wouldn't work on a 30ma RCD fused spur for a whirlpool bath? There was a 30ma RCD on the CU too. I had phase and neutral probes on the load, and earth probe on the incoming neutral, and both RCD's tripped each time. I put it down to it being an older device, but other thoughts welcome!

Two rcds on the same run would need some discrimination not really a good design though in my book.

timhoward · Feb 6, 2021

Pete999 said:
Two rcds on the same run would need some discrimination not really a good design though in my book.

Completely agree. Someone else changed the CU and didn't reconnect what they (understandably) thought was a shower circuit because they couldn't find it. Left a nice sticker saying so too. I found it was feeding the whirlpool and it will be changed to normal spur in due course.
Question was more an academic question about why the normal method of only tripping a downstream RCD might not have worked.

ruston · Feb 6, 2021

If you do get one .- EDS. 54 Silver Lonnen, Newcastle upon Tyne NE5 2HD Have calibration days from time to time.
Give them a ring , they are a good bunch.

pc1966 · Feb 7, 2021

timhoward said:
Question was more an academic question about why the normal method of only tripping a downstream RCD might not have worked.

Ideally a RCD trips on the deliberate test current.

In practice they can trip on switching spikes even when there is no fault. My oven, for example is on a 32A RCBO and maybe once in 6 months or so trips it when switching off. So at least I get my pizza cooked first...

Lucien Nunes · Feb 7, 2021

I'd go so far as to say ALL MFT tests are 'live tests' on the spirit of EAWR (Injection of a charge into a cable them males that cable charged)

Continuity tests inject a few volts with very limited current, so there isn't really a tangible electrical hazard present. The 5V supply in a USB or HDMI cable carries higher risk and a 9V PP3 battery higher still.

Insulation tests provide a high enough voltage to shock, although the current is again very limited, typically 1mA, so unlikely to cause physiological effects. However there is a risk of consequential accident from the shock, such as falling off a ladder.

The risk of life-changing or fatal shock from a 230V mains supply is so much higher, that it is misleading to lump all tests under the same banner as far as risk management is concerned and I don't think that is the intention of the EAWR text.