Electrical Arcing

[T_Dog]

I have noticed that Thai folks are a bit fanatical about unplugging appliances that are not in use. Two days ago I saw why. Had an unused (i.e. nothing plugged into it) expansion outlet plugged into a wall outlet when suddenly there was a pop and a loud 50 hertz buzz. Looked over and saw a 3 inch flame coming out of the wall outlet. Took about 30 seconds to get everything shut down and during the whole while the corona was sustained. Molded plastic plug has a huge hole in it and the outlet has damage from the arcing as well. Appears a high voltage transient or maybe an insect got the arcing started and somehow it was sustained once a bit of carbon/soot path was established.

This got me wondering if anyone is using Arc-Fault Current Interrupters (AFCI) here in Thailand. After seeing this, I am thinking it might be worth it.

[haveaniceday]

I am curious, did you have a RCBO (safety switch) installed?

[bankruatsteve]

Yah - Thai's tend to be supersticious - probably b/c those things happen. And, probably a bug or improper wiring. Never heard of AFCI but if you read this forum, RCD/RCBO is recommended (if you had, you would not have seen that problem).

[InterestedObserver]

Yah - Thai's tend to be supersticious - probably b/c those things happen. And, probably a bug or improper wiring. Never heard of AFCI but if you read this forum, RCD/RCBO is recommended (if you had, you would not have seen that problem).

RCD/RCBOs are not intended to detect an electrical arc that does not involve earth (ground). A L-L or L-N arc will not be detected by a RCD/RCBO.

[Crossy]

The AFCI is a relatively new device, indeed I'm not even sure what they are called in UK terminology. They appear to be designed to detect arcing at loose connections which is a potential source of electrical fires, I'm not sure if one would detect the type of incident described by our OP.

I wonder why the MCB didn't operate, T-dog, what current rating is the MCB protecting this circuit?

A sustained arc of that magnitude requires a significant current to keep it alight, I'm glad all was resolved safely.

[haveaniceday]

I am not arguing, just learning, if this outlet was 3 holed and earthed, and the arcing and subsequent fire touched the Earth, would this likely to have set the RCBO off? T-Dog, was this outlet earthed?

So the easy solution is to install on / off switches for the outlets, like standard in oz, but, does that only remove the issue to the switch???

Or is the switch less likely to have issues? Still, it is another barrier to mishaps, like kids putting things into holes, the outlet is not live when inserting devices in that often cause a spark.

Edited December 21, 2011 by haveaniceday

[T_Dog]

The AFCI is a relatively new device, indeed I'm not even sure what they are called in UK terminology. They appear to be designed to detect arcing at loose connections which is a potential source of electrical fires, I'm not sure if one would detect the type of incident described by our OP.

I wonder why the MCB didn't operate, T-dog, what current rating is the MCB protecting this circuit?

A sustained arc of that magnitude requires a significant current to keep it alight, I'm glad all was resolved safely.

The circuit was on a 16Amp Square D RCD (10mA) circuit breaker. The current had to be less than 16 amps during the arcing I suppose. I'll a photo of the plug once I get a bit more used to the new forum system and have a bit more time.

[Crossy]

The AFCI is a relatively new device, indeed I'm not even sure what they are called in UK terminology. They appear to be designed to detect arcing at loose connections which is a potential source of electrical fires, I'm not sure if one would detect the type of incident described by our OP.

I wonder why the MCB didn't operate, T-dog, what current rating is the MCB protecting this circuit?

A sustained arc of that magnitude requires a significant current to keep it alight, I'm glad all was resolved safely.

The circuit was on a 16Amp Square D RCD (10mA) circuit breaker. The current had to be less than 16 amps during the arcing I suppose. I'll a photo of the plug once I get a bit more used to the new forum system and have a bit more time.

Is it an RCD or an RCBO? An RCD has NO overcurrent detection.

[T_Dog]

I should have said RCD-type circuit breaker to avoid confusion. Yes, it is a 16 Amp trip, (10 mA RCD) RCBO. QOvs RCBO according to the instruction sheet.

[T_Dog]

Here are the photos of the molded plug. The expansion outlet is from HomePro so fairly good quality. It had been stored outside so I suppose it may have been moist but I am still baffled as to what started the arcing in the first place. The Mitsumi outlet it was plugged into fared a whole lot better as the plastic is obviously of higher temperature material.

[haveaniceday]

Oh Yuk....

Sorry I can not help you as I ask the knowledgeable ones here, but just going back to my question post #6, this is a 2 prong plug, if the outlet was 3 holed and grounded, we are talking a fire and "fire works" going snap, crackly and pop here, would it likely trip the RCBO in this case?

[T_Dog]

Oh Yuk....

Sorry I can not help you as I ask the knowledgeable ones here, but just going back to my question post #6, this is a 2 prong plug, if the outlet was 3 holed and grounded, we are talking a fire and "fire works" going snap, crackly and pop here, would it likely trip the RCBO in this case?

I missed your question earlier. The outlet was earthed, but the arc must have only involved the hot and neutral on the plug side. Even 10 amps would have been 2400 watts of dissipation in a very small area. If an arc causes ionization of the surrounding material, odd things can happen that can support high currents as was the case here. Again, this did not trip the 16 amp RCBO.

[Crossy]

Yup, that's certainly nasty, I wonder what triggered it

A thought, have you tested the RCBO with the test button since it happened? It's not unknown for the contacts to stick.

[haveaniceday]

Slightly off subject, is doing a test on each of the RCBO's in the house, (hot water & consumer boxes) one of those things you should test once a month?

I have a bunch of things I do the first of each month, but I never thought about these.

[Crossy]

You should certainly test your RCDs on a regular basis, monthly is probably over the top mind