Act 6 dimmer pack PAT test failure.

[Tomo]

They mean built-in surge suppression, usually Metal Oxide Varistors (MOVs)

They'll clamp a 500V test voltage almost instantly, but remain effectively open-circuit at 250V.

Applying a 500V test voltage will tend to degrade them over time, as they're intended for short-lived events rather than the many seconds that a tester will apply.
The current limit in the tester is supposed to be sufficiently low that they won't be immediately destroyed, but I wouldn't want to rely on that.

They're increasingly likely to be fitted to modern electronic equipment, so a good thing to be aware of.

[David Duffy]

MOVs (metal oxide varistors) are certainly widely used across A-N (after the fuse of course). Class X and Y capacitors are also very common between A-N (also after the fuse). As far as I know the only components that should be across A-E and N-E are class Y capacitors as they are designed not to go short if they fail.

Doesn't an insulation test normally short A & N together and apply the test voltage from the A+N to E ? That way there is no dangerous voltage applied to MOVs or the input circuitry. The caps from A-E and N-E are usually rated for 2kV+, so will not be damaged by the 500V test voltage.

So I've never heard the term "class Y varistor" and don't recall ever seeing a MOV going to earth in any of the hundreds of linear and switch-mode power supplies that I've worked on over the last 30 years.

Maybe some are seeing class Y capacitors and thinking they are MOVs. The light blue ones certainly look similar.

Edited July 7 by David Duffy

[indyld]

When doing IR tests on a device by hand I.e when not using a 'machine that goes beep' but with a MFT, I always connect L+N together. It's also a good test to apply to moving lights when they return and one suspects they may be wet inside. It saves the massive bang and the expensive power supply replacement when it turns out there is water somewhere.

Dunno what a 'pat machine' does, but would make sense to do the same. 

I've not seen a MOV to Earth either.

Edited July 7 by indyld

[Tomo]

Some surge protected devices have MOVs between L&N and PE to prevent damage or danger if PE significantly departs from N - eg from a nearby lightning strike. Might also intentionally trip the upstream protection.

And of course, as the 500V insulation test involves putting 500V between the tied-together LN and PE, this will immediately activate this type of internal protection.

Not common at the moment, but becoming more so.

Seaward FAQ for reference:

https://www.seaward.com/gb/support/pat-testing/283a936-supernova-elite/faqs-and-articles/135-how-to-test-surge-protected-equipment/

[David Duffy]

I understand about specific surge protection devices, but have not come across MOVs going to PE inside any general plug-in devices. Of course larger dimmer packs are on the boundary of that classification  due to the heavy current draw.

[sunray]

On 7/7/2024 at 8:41 AM, indyld said:

When doing IR tests on a device by hand I.e when not using a 'machine that goes beep' but with a MFT, I always connect L+N together. It's also a good test to apply to moving lights when they return and one suspects they may be wet inside. It saves the massive bang and the expensive power supply replacement when it turns out there is water somewhere.

Dunno what a 'pat machine' does, but would make sense to do the same. 

I've not seen a MOV to Earth either.

Just a short while ago I repaired a slim LED PAR64 RGB. The PSU being a 12V SMPS in an ABS enclosure (think laptop supply but flex in and out). As the fault was the main 10V O/P cap ( directly across the 12V) had lost its blue smoke and taken 2 bridge rectifier diodes with it I didn't delve too deeply in the mains end but I was surprised to see 3 light blue smarties which I assumed were all MOV's, 2 brown smarties (which I assumede were caps), a large red cap and 2 chokes. Anoyingly the photos I took only show the ELV end.

[indyld]

@sunray picked a random SMPS out of the bin, plenty of blue smarties that are caps. 

[dave-w]

Success!! PAT tester accepted that the insulation test was inappropriate for the ACT 6 mains input circuitry and passed it on the basis of Earth Leakage which was only 0.8ma.  He also passed our spare.

I had a moment's panic when I inadvertently re-assembled the dimmer with the triac panel flying earth lead connected to the Neutral bus on the PCB.  Fortunately that even tripped an RCD and alerted me before he got to test it again!

Well its been a long and interesting road.  Thanks to everyone for input. I hope the outcome might save someone a lot of time!!

[David Duffy]

9 hours ago, sunray said:

... I was surprised to see 3 light blue smarties which I assumed were all MOV's, 2 brown smarties (which I assumed were caps), a large red cap and 2 chokes.

MOVs are often the same blue colour as the class Y disc capacitors. Dark grey / black discs are usually NTCs (inrush current limiters) but a single phase input power supply should only have one of those, usually fairly close to the input terminals.

Good to hear you got a good result @dave-w. 🙂

[sunray]

10 hours ago, indyld said:

@sunray picked a random SMPS out of the bin, plenty of blue smarties that are caps. 

Yes of course and thinking about it; the caps I used in the bluecomm intercom look just like those.🙄

[KevinE]

Interesting that my post about class Y MOVs is doubted. I have had several items in for repair that have come as a result of a PAT fail, all due to insulation resistance and all due to live/neutral to earth MOVs being triggered. When I first saw them I had to download the datasheet to be sure what they were. I think they were epcos (formerly siemens) who are now TDK.

 

Here is a datasheet from a similar Panasonic device.

 

 

Edited July 11 by KevinE
found a clearer pic

[David Duffy]

Ah, I've not seen those used from A or N to PE before. After seeing the datasheet I know what you were talking about now. The term "class Y MOV" threw me, but makes sense with the inbuilt fuse so that it faults open circuit. What sort of equipment were they in?

[KevinE]

I think the last ones were in the SMPSU for some profile projectors aka shadowgraph micrometers