Understanding Electrical Safety: The Role of Grounding in Circuit Protection

[Svein]

To begin: This is not a question, but an anecdote.

The other day my wife called to me from the floor above: "Svein, there is no light". OK, I checked the fuses, and sure enough one fuse had tripped. I resat it, only to hear "Svein, it is blinking!". I went up to see what she meant by that and, sure enough, one ceiling light was blinking like a distress signal. "OK" I said. "Turn it off, and I will take a look". "It is turned off, but it still blinks!" So I disconnected the lamp and started trying to find out what had happened. It turned out that she had sprayed the lamp with a cleaning solution in order to clean it for Christmas.

Thinking about it, I arrived at a conclusion. Soapy water plus dust makes a high impedance resistor. The metal in the lamp was grounded. The light switch was a one-pole type, so one lead was "live". Thus the LEDs were able to draw a small current. This current apparently charged a capacitor in the LEDs and when the capacitor voltage passed a threshold, the LEDs turned on and promptly discharged the capacitor. After this behavior had gone on for a minute, the RCD tripped and the whole floor went dark.

I took the lamp apart and saw that the voltage distribution center was fairly wet (as I had suspected). Worse, the guys in the lamp factory had obviously discovered some bad insulation somewhere and had wrapped the distribution points in electrical tape. When I touched the tape, it promptly fell off. Some strong language followed, but I fetched some shrink tubing and a heat gun, dried out the thing and put shrink tubing over the wires. Case closed.

 

[jedishrfu]

Nice! She may be telling you that she wants YOU to clean the lights in a subtle way. :-)

 

[NTL2009]

A similar thing can happen when you use a CFL/LED on a dusk/dawn switch or timer intended only for filament bulbs. The electronic switch has a bit of leakage, charging the cap as you describe, and the light flashes on briefly, and cycles again when the cap can charge up.

 

[Guineafowl]

Maybe spray onto the cloth, then wipe the lamp, rather than soaking the lamp?!

It was an earthed/grounded lamp, so shouldn’t the plug be polarised, hence truly ‘off’? Or was it capacitive coupling from the neutral?

 

[Svein]

It was an earthed/grounded lamp, so shouldn’t the plug be polarised, hence truly ‘off’? Or was it capacitive coupling from the neutral?

Well, as I have said in various posts, Norway's traditional power distribution does not include "neutral". It is a "delta" distribution:

The "ground" is not a part of the distribution, it is named "protective ground" and it is connected to "earth" at the base of the house. This ground is theoretically at the center of the delta, but in practice it may be quite off center electrically. Anyhow, it is quite possible to draw some small current between either phase and "protective ground". And, since no phase is more "neutral" than others, all wires are "live" (that is why double pole light switches are mandatory in "wet" rooms and outside lights).

 

[Guineafowl]

Well, as I have said in various posts, Norway's traditional power distribution does not include "neutral". It is a "delta" distribution:
View attachment 217180
The "ground" is not a part of the distribution, it is named "protective ground" and it is connected to "earth" at the base of the house. This ground is theoretically at the center of the delta, but in practice it may be quite off center electrically. Anyhow, it is quite possible to draw some small current between either phase and "protective ground". And, since no phase is more "neutral" than others, all wires are "live" (that is why double pole light switches are mandatory in "wet" rooms and outside lights).

Ah, I didn’t know that. But how is the protective ground part of the circuit? The three phases off the delta look like they are not referenced to it. How does a fault current get back to the supply?

 

[Svein]

Ah, I didn’t know that. But how is the protective ground part of the circuit? The three phases off the delta look like they are not referenced to it. How does a fault current get back to the supply?

Through the earth. But due to the geology in Norway the "earth" does not conduct very well, so even a small ground current can create a large voltage drop between the supplier and the ground fault.