Reverse Engineering a GFI Plug

RE: Reverse Engineering a GFI Plug

three-phase — Fri, 29 Dec 2017 17:10:19 GMT

Good write up John.

For those wanting more information on these types of devices, Beama produce a good guideline, all be it biased towards the UK market and BS7671.

Beama - The RCD Handbook

Nema produce a similar booklet for the North American market;

Nema - GFCI Booklet

The 30mA setting for RCDs was based around work by Gottfried Biegelmeier in the 1950s that showed 95% of the population could withstand up to a 40mA shock under dry conditions. His work was then developed into IEC60479 that is utilised for protection against touch and step potentials in HV substations along with IEEE 80 Standard. I believe however, that the North American market go down to 6mA on their GFIs.

They are not the most reliable of devices. Fixed RCDs are recommended to be tested every 3 months using the built in test button. RCD plugs are recommended to be tested every time they are used.

Ramp testing and timing of the devices is carried out during condition assessments of installations. Recommended frequency for domestic market is 10 years, industrial and commercial 3 to 5 years and special locations 6 monthly to yearly. Specific devices for RCD testing are available such as this Megger device. More often now a days they form part of a multi function tester that is utilised to test all aspects of an electrical installation as pointed to by [mention:b0bc65b9ecdc4307bd967592f00e340a:e9ed411860ed4f2ba0265705b8793d05] in his review of the Megger Insulation Tester.

Kind regards

RE: Reverse Engineering a GFI Plug

ntewinkel — Sat, 23 Dec 2017 18:40:40 GMT

> Unfortunately the way this GFI failed would not have protected the user in the case of a real emergency.

This reminds me of how, in engineering, we aim for "how does it work" and often forget to ask the most important question of all: "How does it fail?"

RE: Reverse Engineering a GFI Plug

DAB — Sat, 23 Dec 2017 18:13:01 GMT

Great way to seize victory from the jaws of failure.

I never gave the GFI much thought, but your excellent description of the circuit made sense to me.

The circuit is very simple, but as you found out, not very robust under component failure conditions.

Great job.

DAB

RE: Reverse Engineering a GFI Plug

genebren — Sat, 23 Dec 2017 17:09:31 GMT

Good testing of a product that should not have failed. I never press the trip/test buttons, but now I will have to re-examine that behavior.

RE: Reverse Engineering a GFI Plug

shabaz — Sat, 23 Dec 2017 11:35:57 GMT

Hi John,

This is very interesting - glad that all is well and you were around to unplug and examine the item. I too have RCDs for some devices, and so after seeing your analysis and reverse engineering, and [mention:a5312e9762dd4699ba5d3876da34ffe5:e9ed411860ed4f2ba0265705b8793d05] comment, it is now clear that one should be prepared for that condition when the test button is pressed. Ordinarily I would not have given it a seconds thought, I'm pretty sure the user instructions do not advise in such detail.

I tried to examine the plug-in RCDs here briefly just now - they have security screws so I didn't open them (I don't have a spare one).

RE: Reverse Engineering a GFI Plug

michaelkellett — Sat, 23 Dec 2017 09:51:17 GMT

My guess is that the solenoid coil is only rated for a very short operation, during which it should pull the switch and remove power from itself. If the switch jams not only do you run the risk of electrocution but have the fun of being set on fire as well.

There are two lessons here,

1) test these things often

2) be ready for issues when you test

I'll have a look around and see if I have any I can pull apart to see if they are similar.

BTW, in the UK we call them ELCBs (Earth Leakage Circuit Breaker) or RCDs (Residual Current Device).

Happy Christmas