Interesting read...loop testing and LED Dimmers

[Leesparkykent]

Electrical test equipment | power station to plug | Megger - https://uk.megger.com/electrical-tester-contractor-edition/april-may-2017/loop-testingand-led-lamps-what-s-going-on

 

[The Ghost]

Yeh read that, interesting and informative. More arrows in the quiver troubleshooting!

 

[Marvo]

Useful article.

Might be nice to start a thread in the Trainee forum for posting articles on various anomolies encountered during different test proceedures. This one would be a good one to open with. I'll see if I can find some articles on MOV's in switch mode supplies having an effect on IR test results and capacitive and inductive coupling causing ghost voltage readings on redundant cables to start with as well.

 

[westward10]

Thought this would be obvious, the dimmers that is.

 

[Marvo]

Possibly obvious to many but it's the kinda thing that probably doesn't get covered thoroughly in a classroom nowadays so I'm sure it would be of benefit to some as well.

Dimmers especially seem to cause confusion, the voltage waveform from a dimmer can also give weird readings on testers as well because of its rapid rise and decay and often altered phase angle.

 

[davesparks]

Carry on this train of thought and you get to the realisation that dimmers affect the behaviour of the circuit under fault conditions. A fault downstream of a dimmer will probably not cause operation of the ocpd within 0.4S as a result of the current limiting effects of the dimmer.
Maybe a minor concern with the standard switch plate dimmer.
But what about a centrally located dimmer system? Will the ocpd operate in the required time on a fault between dimmer and load?

 

[Leesparkykent]

Useful article.

Might be nice to start a thread in the Trainee forum for posting articles on various anomolies encountered during different test proceedures. This one would be a good one to open with. I'll see if I can find some articles on MOV's in switch mode supplies having an effect on IR test results and capacitive and inductive coupling causing ghost voltage readings on redundant cables to start with as well.

yeah that would be a good thread for the trainee section. I'm back on Monday so could start it then as struggle on the phone unless you want to start it in the mean time ;-)

 

[westward10]

Carry on this train of thought and you get to the realisation that dimmers affect the behaviour of the circuit under fault conditions. A fault downstream of a dimmer will probably not cause operation of the ocpd within 0.4S as a result of the current limiting effects of the dimmer.
Maybe a minor concern with the standard switch plate dimmer.
But what about a centrally located dimmer system? Will the ocpd operate in the required time on a fault between dimmer and load?

Your standard dimmer will not restrict fault current as the triac should fail to the close position allowing fault current to pass.

 

[marc8]

this may not be what everyone does & only as we undertake a bit of new build work, we use click connectors or similar & we don't install lamps until after ZS same with Dimmers wago out or stick a temporary switch on, finish testing & then install lamps & dimmers. also helps with cash flow fitting a click & not having an expensive down light on the site that might be fitted for 40 days or so before final testing.
someone did mention to me a few years ago carry out r1&r2 & then calculate ZS which is fine but where possible we undertake ZS testing as part of our job.

 

[Midwest]

Might be obvious to some, but it wasn't to me, when I encountered this issue last year. Installing some LED down lights, replacing old 12v ones. I had removed the old ones, and did my testing and got crazy Zs result. After lots of head scratching and dismantling the circuit, I removed the dimmer and got a decent Zs. Lesson learnt.

Wished I'd read that article sooner.

 

[Pat H]

When I did my 2394/95 training that was always one of the first things before testing identify vulnerable components. Dimmers, shaver points, USB supply 13A outlets. neon lights (cookers showers heaters etc)
Dimmers replaced with normal manual switches for testing.

However I did get caught out a while ago with a rogue 13A socket outlet in a RFC that had an RCD built in one of the sockets. Odd IR results until I discovered it.

 

[Specialist]

Good read, thanks Lee.

 

[davesparks]

Your standard dimmer will not restrict fault current as the triac should fail to the close position allowing fault current to pass.

That all depends on what you are calling a standard dimmer.
But if a fault occurs with the dimmer set to output at 25% how long will it take until the ocpd clears the fault?