# MCB Does not Trip on short circuit.

#### LesJones

Joined Jan 8, 2017
3,140
As post # 17 mentions a 25 A MCB at the supply end of the 50 meter cable run I think it is VERY unlikely that two MCBs are faulty.
When you say in post #1 that a cable smokes is that the cable that you THINK is 4 sq mm or some other cable ? Can you post a diagram showing everything between the incoming supply to your house to where you short live to neutral. Also could you do this test. In your garage monitor the mains voltage as close as possible to a socket where you can plug in a reasonably heavy load such as a 3KW electric kettle. (This will take about 12 amps.) Note the voltage reading before you switch the kettle on and again when the kettle is switched on. This should allow us to calculate the resistance between your garage and the incoming supply. (I will assume that the impedance of the incoming supply is low enough to be ignored.)

Les.

#### JustMe234

Joined Feb 25, 2017
68
In the circuit we are discussing things are connected in this way.
RCD(GFCI) - Then several MCB different amperage for plugs, lights, welder, air compressor etc. Welder and compressor has his own MCB.
I change the test wire to thickest i can find 6mm and finally MCB trip after 5-7 seconds.

Also did the other test:
I live in rural area and sometimes quality of electricity is shitty.

Here are some other specs of RCD and MCB.

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#### LesJones

Joined Jan 8, 2017
3,140
If we assume 2.9 kW is about 13 amps and that drops the voltage by 35 volts (200 - 165) the the supply impedance is
35/13 = 2.7 ohms. So 200 volts across 2.7 ohms is 200/2.7 = 74 amps. This is not high enough for the magnetic trip to be triggered (Which is fast tripping which happens above 10 x the current rating for a curve C MCB) but it is high enough for the slow tripping caused by the thermal trip. This is a link to the characteristics for Legrand MCBs
I think this solves the mystery.

Les.

#### JustMe234

Joined Feb 25, 2017
68
Thank you for valuable information. I will keep in mind those calculations. Now i need to test RCD with the resistor if i find it.

What can be best practice in these conditions? My concern is for some reasons might be a short somewhere or a thin wire, overheat, lose connections and start a fire.

#### LesJones

Joined Jan 8, 2017
3,140
Pressing the test button on the RCD is just the same as connecting a resistor between live and earth so thaat is all you need to do to test it. As said in several posts the RCD DOES NOT TRIP due to excessive current between live and neutral. Just as a matter of interest is the earth in your garage provided by ground rods ?

Les.

#### MisterBill2

Joined Jan 23, 2018
8,486
In a distribution panel there can be ONLY one Main Circuit Breaker. All of the others are branch circuit breakers with their feed being controlled by that MAIN circuit breaker.
And I see in the photo that the markings on the breakers are intentionally obscured, and that is a serious puzzle. Likewise on the first pair of photos.
AND, ONCE AGAIN, a ground leakage protection device will not provide any overcurrent protection. It seem that there i a bit of confusion here. A single Main Circuit Breaker, (MCB) is used to protect the portion of the system between the service wattmeter and the distribution panel, at least here in the USA. Having a leakage-to-ground detection device in this line assures that in the event of ome ort of ground-fault defect all of the power will be switched off, leaving the entire place in the dark and providing no clue at all as to where the fault i located.
For a while, here in the USA, some localities required arc-fault-detection circuit breakers, until it was proven that some kinds oof appliances would cause them to trip repeatedly when there was no fault present.

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,684
My concern is for some reasons might be a short somewhere or a thin wire, overheat, lose connections and start a fire.
Agreed. Your feed to the panel in the workshop/garage should have a breaker that protects the wiring from the main panel to the garage. That breaker should have blown, otherwise the wire protects the fuse.

Remember the old joke where the transistor blows to protect the fuse.

I have an APC power strip where the thermal fuse blew to protect the components that should be stopping the surge which is stupid.

The RCD can be used as a disconnect in the garage.

===

At my house, the pole mounted transformer feeds 4 properties. two properties have had service upgrades and the transformer has been replaced 2x in 60 years. It has a fuse on the primary, probably 9600V.

There is no protection on the secondary to the meter and from the meter socket to the main "fuse". I still have fuses. it's a pull out cartridge fuse. I THINK the new electronic meters can disconnect the service remotely and I THINK that the two properties that were upgraded have disconnects installed after the meter. In case of fire, it would be a good thing. Gas can be shut off from the outside with a wrench. Water can be disconnected from outside with a 5-sided socket wrench for access and a long T-handle wrench to move the valve.

#### MisterBill2

Joined Jan 23, 2018
8,486
Agreed. Your feed to the panel in the workshop/garage should have a breaker that protects the wiring from the main panel to the garage. That breaker should have blown, otherwise the wire protects the fuse.

Remember the old joke where the transistor blows to protect the fuse.

I have an APC power strip where the thermal fuse blew to protect the components that should be stopping the surge which is stupid.

The RCD can be used as a disconnect in the garage.

===

At my house, the pole mounted transformer feeds 4 properties. two properties have had service upgrades and the transformer has been replaced 2x in 60 years. It has a fuse on the primary, probably 9600V.

There is no protection on the secondary to the meter and from the meter socket to the main "fuse". I still have fuses. it's a pull out cartridge fuse. I THINK the new electronic meters can disconnect the service remotely and I THINK that the two properties that were upgraded have disconnects installed after the meter. In case of fire, it would be a good thing. Gas can be shut off from the outside with a wrench. Water can be disconnected from outside with a 5-sided socket wrench for access and a long T-handle wrench to move the valve.
KSS, on your plug strip you will probably find that the surge protector goes into a short circuit mode when there is a big enough transient spike. Mostly they stay that way until the current is removed. On many occasions they simply stay short circuited. I have experienced that more than once, that the surge protector simply short circuits and stays that way.
Now for the power to the garage, there really needs to be an overload device at the main panel, where that wire is powered, sized to protect that wire. So if the circuit is our #10 wire code calls for a max of 30 amps protection, while for #8 wire I think a 50 amp breaker is OK, although it may only be a 40 amp breaker. For an arc welder as the only 50 amp load, given the short cycle of most arc welding, 50 amps will be safe for most arc welding styles.

And for the mains protection, I favor the cartridge fuse/holder arrangement because it is easier to find in the dark, and firemen know how to pull a fuse without needing to stop and think. And when you pull it then it will stay off until you put it back.

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,684
KISS, on your plug strip you will probably find that the surge protector goes into a short circuit mode when there is a big enough transient spike. Mostly they stay that way until the current is removed. On many occasions they simply stay short circuited. I have experienced that more than once, that the surge protector simply short circuits and stays that way.
That's what I would expect. This was an APC surge suppressor power strip. The thermal fuse blew, the one that opens based on temperature. The breaker on the strip didn't trip. The outlets stayed powered.

The LED that said it was unprotected lit (or whatever). When you shorted out the thermal fuse, the indicators said it was "protected".

I doubt anyone makes a thermal fuse that shorts when the temperature exceeds some value.

Usually the ZNR's short which trips the breaker on the strip. Not in this case. It wasn't my strip to begin with, so I don't know what happened.

I did see the results of a ISOBAR that was probably hit by lightning and the connected equipment warranty was honored. It was all black with soot.

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,684
And for the mains protection, I favor the cartridge fuse/holder arrangement because it is easier to find in the dark, and firemen know how to pull a fuse without needing to stop and think. And when you pull it then it will stay off until you put it back.
These are tamper evident closures (meaning they have to be cut) to open the box. The disconnect is outside the home just under the meter.

Joined Jan 15, 2015
5,916
And I see in the photo that the markings on the breakers are intentionally obscured, and that is a serious puzzle. Likewise on the first pair of photos.
Leaves one to wonder? Why important information is being intentionally omitted?

Ron

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,684
Breakers have always been that way. There is so little room to write anything on the surface seen. So current makes sense and something to designate it as an RCD makes sense.

The IΔm even makes sense to me. It's crypic, but not the way my mind works. Believ me, it does not work like everyone else's.

I would normally associate m with mass like in Newtons Law F=mA

So iΔm=1000A ; now the question is what's "m" ; IΔm means interrupting capacity. Hopefully it's on the datasheet as that.

You have identified it as a Car. Probaby the next most important thing to know about the car is what fuel it uses. The diesel nozzle doesn't fit in a newer car tank and you won't put electricity in a gas tank. People are stupid. You need to know where to put the gas. dual tanks on the rear (jaguar); Behind one of the taillight. Hidden behind the license plate? Now, it seems to be on one place and either the right or left side of the car.

manual or automatic transmission, you need to know that too. Where is the shifter and where are the gears?
What happens when your confronted with a pick-ip with 3 on the tree for the first time. No labels. You have to figure out where the gears are. Confronted with an 18 wheel tractror-trailer for the first time when I was about 20 YO. My cousin asked me if I wanted to take it around the parking lot? I took it for a spin. 18 gears, 2 shift levers. Had to double-clutch everything. Clutch to get in gear. Clutch to get out of the gear. The thing tht got me was, the brake pedal "looked" like an automotive accelerator. Luckily I had room to stop.

Then, there is how to start the thing. keyfob+button, on the floor, on the column, on the dash.

Where and how to operate the emergency brake. Well it's not even called an emergency brake anymore, is it? It's a parking brake. Ever drive a car 10 miles without hydraulic brakes? I have.

All you need to know is that its an RCD or a breaker and what circuit it belongs to. The designer and repair person needs the model number and sometimes the brand.

#### MisterBill2

Joined Jan 23, 2018
8,486
These are tamper evident closures (meaning they have to be cut) to open the box. The disconnect is outside the home just under the meter.
My purpose is not to catch tampering, it is to keep the power off while I AM WORKING ON SOMETHING. And it is simple to plug the fuse holder back in when I am done. The tamper-evident thing is not so simple to undo.

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#### KeepItSimpleStupid

Joined Mar 4, 2014
4,684
I agree. I was talking about your house burning and the fire company has to cut power.

We are not talking about a lock out/tag out system. Multiple locks on the "hasp". Only when everyone is done, the lock can be removed.

I;'ve done the take the fuseholder cap with you. Unplugging from the wall and a sign was good enough where I worked. e.g. removing a 60A 3-phase plug.

Although a few systems had keys. The key was in the panel all the time unless it was being worked on. That time frame might be a month or so.

#### JustMe234

Joined Feb 25, 2017
68
Just as a matter of interest is the earth in your garage provided by ground rods ?
Yes there is a galvanized rod about 1.5 meters in ground.

Agreed. Your feed to the panel in the workshop/garage should have a breaker that protects the wiring from the main panel to the garage. That breaker should have blown, otherwise the wire protects the fuse.

Remember the old joke where the transistor blows to protect the fuse.
Probably the wire was thick enough to handle it. I found some fuse holders and they can be installed in din rail also. Have to look the amperage because the welder will blow them quickly.

#### LesJones

Joined Jan 8, 2017
3,140
Some points on the discussion. In one post the TS says there is a 25 amp MCB at the house end of the cable to the garage so that will protect the cable to the garage.
DIN rail mounted MCBs are available in 6 amp rating. B curve MCBs are the most common in domestic applications.
As the voltage in the garage is only 200 volts off load and drops to 165 volts with a 3KW load using the compressor and welder could be a problem as we have not been told any information on either device.

Les.

#### MisterBill2

Joined Jan 23, 2018
8,486
Some points on the discussion. In one post the TS says there is a 25 amp MCB at the house end of the cable to the garage so that will protect the cable to the garage.
DIN rail mounted MCBs are available in 6 amp rating. B curve MCBs are the most common in domestic applications.
As the voltage in the garage is only 200 volts off load and drops to 165 volts with a 3KW load using the compressor and welder could be a problem as we have not been told any information on either device.

Les.
A point of naming here! If it is not the first breaker in the sequence it is not the main circuit breaker, just a "CB" in the sequence. So the rest are not "MCB". That can be a confusing thing.

#### JustMe234

Joined Feb 25, 2017
68
DIN rail mounted MCBs are available in 6 amp rating. B curve MCBs are the most common in domestic applications.
As the voltage in the garage is only 200 volts off load and drops to 165 volts with a 3KW load using the compressor and welder could be a problem as we have not been told any information on either device.
Early in morning i get 240V-230V in that time i work with the welder. With loads voltage might drop to around 225V. Air compressor has a max 1.5KW and the welder max output around 140A.
Should i go for fuse box or stay with this configuration?

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#### Alec_t

Joined Sep 17, 2013
12,009
A point of naming here! If it is not the first breaker in the sequence it is not the main circuit breaker, just a "CB" in the sequence. So the rest are not "MCB".
Maybe not where you are, but they are in the UK at least. 'MCB' here stands for 'Miniature Circuit Breaker'.

#### MisterBill2

Joined Jan 23, 2018
8,486
OK, with the arc welder the current of interest is the mains current, which should be much less than the arc current. And now the part about MCB= MInature circuit breaker makes some sense. A 6 amp MCB would be adequate for a hand drill or even a big soldering iron, it would be very marginal for a half HP air compressor even at 240 volts.

The small arc welder that I once borrowed needed 30 amps at 240 volts, and that was rather heavy wiring, #10 (USA size) wire, and that was marginal.So there may be a wire size problem. There are charts and tables for wire sizes versus current ratings and loads.