Plugtop RCD Adaptor test problem

Today I have three 13A 30mA RCD plugtop adaptors to test - two around 5-7 years old and one brand new. When testing these I insert a Seaward NTB1 isolation transformer to prevent tripping any upstream CPDs.

The two old adaptors would not energise when connected to the isolation transformer, however they would energise when connected direct to the mains (they were both immediately tripping, indicated be the repeated 'clicking' sound when connected to the isolation transformer.) However the new adaptor energised as normal when connected to the isolation transformer.

I then took the questionable two adaptors and tested them connected direct to the mains and they both gave good readings, tripping within acceptable times for I and 5I at 0° and 180° (also - as expected - tripping an upstream RCD).

Tests carried out with two separate instruments, a Megger MFT1741 and a Kewtech KTD50, gave the same results.

This has left me confused...
- if the isolation transformer was at fault, why would the third (new) adaptor energise?
- if there is another problem with the two old adaptors, then what could that problem be, since they give credible results when tested without the isolation transformer?

Any help & suggestions welcomed.

Two RCD's in 'series' as it were will always give problems.
Is the upstream rated at 30mA as well?
Manufacturing differences, ambient temperature, other external influences can affect the operation. One might trip at 29mA, the other at 31.
Can the upstream RCD be removed? If not, why use plug in adaptors, the protection is there.

I cant answer what might be wrong with the transformer, but you test RCD's so that they function properly in everyday use, and in everyday use they will be plugged into the mains, not through a transformer.

littlespark said:

Two RCD's in 'series' as it were will always give problems.

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Agreed - this is why I have the NTB1 isolation transformer. I test several hundred of these RCD plugtop adaptors using it every year without problems.

littlespark said:

Can the upstream RCD be removed?

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No, that is on the main board for the office & workshop

littlespark said:

If not, why use plug in adaptors, the protection is there.

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We don't use the adaptors, they belong to the customer that has brought them to us for ISITEE, along with other leads and equipment.

littlespark said:

I cant answer what might be wrong with the transformer, but you test RCD's so that they function properly in everyday use, and in everyday use they will be plugged into the mains, not through a transformer.

Click to expand...

As above, the transformer is only used whilst testing, to prevent the board to the office & workshop repeatedly tripping.

Yeh. I thought about it after I posted. Testing RCD adaptors from another location.
So you've done hundreds of these tests with the transformer and these are the only problem ones?

Install a test socket without RCD protection. Label it accordingly.
Or just say the adaptors failed and tell client to get new ones.

I am going to 'Pass' them on the basis they gave good readings to all 6 tests (½I, I and 5I) when connected to the mains, which as you rightly state, is how the customer will be using them for everyday use.

For my own sanity, I'm trying to fathom why they won't energise when connected to the isolation transformer, when the new one, incidentally by the same manufacturer (SMJ) will energise.
We stock them for resale and I have opened another two new ones and they energise on the isolation transformer, as expected. Just these rogue two that don't!

Maybe the older ones are also looking at the earth, like some RCBOs

Maybe check the transformer output is still around 230V too.

The older adaptors may have neutral / earth voltage detection that is preventing them operating if the output from the NTB has any deliberate series impedance between the secondary neutral and earth. I've no idea what is in those little boxes, never felt the need to use them because I always test RCDs load-line-to-supply-neutral which does not risk tripping any upstream protection.