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BANGKOK 17 February 2019 03:23
giddyup

Why didn't the circuit breakers trip when I cut the extension cord??

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Was cutting a hedge a couple of days ago and inadvertently cut the electric extension cord with the cutter. Pretty stupid I know as I'm usually pretty careful with the electric cutter and mower, and been using them for nearly 7 years without a problem, but my question is, why didn't the breakers trip? The cord was almost severed and sparks flew, I guess the cutter must be well insulated as I'm still here to tell the story, but a bit of a worry that the power wasn't cut. I have had the breakers trip when a light shorted out, so I know they have worked on some connections, but I was plugged into an outside socket which shouldn't have made an difference, right?

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Not sure if the cutter is in working order again. If not it could be that there is a fuse inside the plug or inside the cutter housing itself. Maybe that fuse/breaker (if present) was faster compared to the one on the mainboard.

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1 hour ago, Carib said:

Not sure if the cutter is in working order again. If not it could be that there is a fuse inside the plug or inside the cutter housing itself. Maybe that fuse/breaker (if present) was faster compared to the one on the mainboard.

The hedge cutter is still working fine.

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5 hours ago, giddyup said:

The hedge cutter is still working fine.

If still in working order I go along with Crossy`s theory.  

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3 hours ago, Crossy said:

Does the breaker have a "Test" button? Press it!

 

If no test button it's a plain MCB rather than an RCBO, a simple cord cut (a momentary short) may not pull enough current for long enough to open it. A 20A breaker (normal outlet circuit) needs 100A to open "instantly" and will carry 25A pretty much forever.

No test button that I see.

P1050687.JPG

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7 minutes ago, Crossy said:

Indeed, a row of regular MCBs.

 

Assuming that you don't have a separate Safe-T-Cut device you have zero earth leakage protection. If you had picked up the live end of the cut cable you quite likely wouldn't be posting here.

 

I strongly recommend that you either install a separate Safe-T-Cut RCBO or (slightly cheaper if you can find one locally) replace that 50A incoming MCB with an RCBO.

 

As you have found, you can get a very satisfactory flash from a brief short which doesn't open an MCB.

 

I'll pop this over to the electrical forum.

Thanks for the info. I guess I could just buy a separate breaker box just to use on the extension cord when gardening? I know some extension sockets have a fuse built in, would that serve the purpose? What does it take to trip a breaker on the fuse box I have now, seems like there is no built in safety at all.

Edited by giddyup
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From your description, it sounds like the short didn't last quite long enough to trip the breaker, but that's not the biggest problem here. 

 

Regular breakers are intended to prevent the house wiring from overheating and starting a fire. They don't offer much protection to you personally. Electricity that is being used around water or outdoors should flow through special ground fault interrupter breakers. So, in our house there are GFI breakers for each water heater and on every circuit that has an appearance outside the house. That includes outlets as well as hard-wiring to a/c units and water pumps.   

 

These individual breakers are faster and more sensitive than the Safe-T-Cut devices which are wired in ahead of the breaker panel. 

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1 minute ago, mahjongguy said:

From your description, it sounds like the short didn't last quite long enough to trip the breaker, but that's not the biggest problem here. 

 

Regular breakers are intended to prevent the house wiring from overheating and starting a fire. They don't offer much protection to you personally. Electricity that is being used around water or outdoors should flow through special ground fault interrupter breakers. So, in our house there are GFI breakers for each water heater and on every circuit that has an appearance outside the house. That includes outlets as well as hard-wiring to a/c units and water pumps.   

 

These individual breakers are faster and more sensitive than the Safe-T-Cut devices which are wired in ahead of the breaker panel. 

Sounds like it would be a big job having to get the existing fuse box replaced?

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2 minutes ago, Crossy said:

The extension cord fuse is there to protect the cable not the user.

 

If you can get an extension with an RCBO built in then that would at least protect you If you cut the cord again.

 

The ideal fix is to replace the 50A breaker with one of these https://www.schneider-electric.co.th/en/product/download-pdf/QO250C10RCBO30 which will prevent you from dying in the shower too 🙂

 

W1siZiIsIjIwMTYvMTEvMTcvMTUvNTUvMzkvNTM5

 

2,700 Baht from HomePro, probably cheaper from your local electrical man.

 

I assume I need a qualified electrician to install it?

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