MCB Does not Trip on short circuit.

Thread Starter

JustMe234

Joined Feb 25, 2017
68
Hello,
In my "Garage Lab" i am using this MCB to add some protection on my electrical wiring. Recently i decided to do some tests because those never trip, talking about heavy loads: Welding, grinder even when lightning strike those don't trip. I run a short circuit on purpose and those stayed on and the only thing i hear was an hummm sound and smoke of course from cables. Running voltage 230V 50Hz.
 

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Thread Starter

JustMe234

Joined Feb 25, 2017
68
Yes the lever works perfect cuts power immediately. I believe that either might be defective or they need over current for long period of times but still a short circuit should trip.
 
The first breaker is an RCD, It WILL NOT trip on an overload. Make sure it's wired right. It should trip when you hit the test button?

The second breaker should trip unless your using too small of gauge wire to test.

I'm assuming you live in a country where the supply is single phase 230 VAC and a Neutral.

The second is a 20A C-curve breaker. https://www.c3controls.com/white-paper/understanding-trip-curves/
You need 5 to 10x (20A) for an instantaneous trip.
Curves are here: http://documents.portalink.net/product/322543/F01573EN-01.pdf
 

Thread Starter

JustMe234

Joined Feb 25, 2017
68
If i am reading the datasheet right my RCD will trip at 300A for at least 2 seconds. Connections looks goods, when the Test button is pressed the RCD trip as it should.

For the test I use 2.5mm(10 gauge I think) wire. Maybe the wire is thin. But i have seen different Breaker brand trip with bad tools(cords).

Yes the supplier provide us with 230V and a neutral.
 

LesJones

Joined Jan 8, 2017
3,132
Is your garage supply fed directly from the output of your meter or from an MCB in the main house consumer unit ? (I am guessing that you are in the UK as your profile does not give any information on your location.) How long (Approx) is the cable run between you garage and the meter or main consumer unit and what is the cross sectional area of the conductors in that cable ?
Les.
 

Thread Starter

JustMe234

Joined Feb 25, 2017
68
Garage is supplied directly from power meter. I live in EU.
Length from power meter to distribution board in garage is around 50 meters and i am not sure about the thickness of the cable but it might be 4mm wire.
 

LesJones

Joined Jan 8, 2017
3,132
IF the cable conductors are 4 sq mm then that has a resistance of 0.0046 ohms per meter. As you cable run is 50 meter that is 100 meters of conductor so the total resistance of the cable is 0.0046 x 100 = 0.46 ohms. That would give a fault current into a short circuit of about 500 amps which should cause the MCB to trip very quickly. I think your MCB is faulty.

Les.
 
I wouldn't really worry about the MCB tripping at high currents. That's really not what it's designed to do.

if you want to trip it: Use 40 mA; R=230/40mA <= 5750; 5.6K is common, I believe. So, you;ll need a 5600 ohm 5-10W resistor.
Connect it between line and ground and it should trip.

Like the US, you keep your ground and neutral separated at the remote box, which I think this is.

In the US, IF it's a detached structure, you need to add a ground rod to ground.

The feed, where power originates, should be protected with a breaker to protect the wire.

In the US we have something called a "main breaker" panel and a "main lug panel".

The "main lug" panel wants to be fed from a remote breaker which acts as the disconnect. Neutral and ground are separate terminals. You would have N, G and L. All separated. The N-G bond is made at the main panel.

The "main breaker" panel can usually be "configured" to that the "main breaker", say 200 Amps acts as a disconnect. It;s permissable to use a 60 amp breaker at the main panel, wire "rated" for 60A and have a 200 A disconnect.

You buy a "ground bar kit" which gives you a place to wire ground too. The panel itself has a means to remove the Neutral to ground bond.

I think something is wrong with your application. The feed breaker should trip if you short the leads in the garage.

We generally don't do RCD's for the main power. The GFCI's and AFCI's are for each circuit and are usually combined with a 15 or 20A breaker for typical 120 V circuits.

We have split-phase. The house is supplied with 120-0-120 center tapped transformer which is grounded and connected to neutral at one point.
 

MisterBill2

Joined Jan 23, 2018
8,446
It sounds like post #13 is talkig about tripping a GFCI, not a current overload type of trip, but a ground fault trip. Tht is totally different. And breakers do fail in ways other than welded contacts.
 
Go back to post #9 where the TS states:

If i am reading the datasheet right my RCD will trip at 300A for at least 2 seconds. Connections looks goods, when the Test button is pressed the RCD trip as it should.

It sounds like post #13 is talkig about tripping a GFCI, not a current overload type of trip, but a ground fault trip. Tht is totally different.
Totally agree. I would test for a 300A trip. Contact resistance also shows up. What the TS then has is a 'magic smoke" generator.

if you really want to test for tripping time, design instrumentation that tests trip time. A piece of wire won't cut it.

At work we had a transformer that was rated at 3000A at 6.3V. Do I even want to test that the transformer delivers 3000A. Nope.
 

MisterBill2

Joined Jan 23, 2018
8,446
One thought is that in a single breaker panel, such as most residential ones, the only portion between the main breaker and the branch breakers is that section of busbar down the center, so that it is unlikely that the main would ever be tripped except by a lightning hit or similar event. And lightning does have enough capacity to force a whole lot of current to flow through even #6 wire for long enough to trip the breaker.
 

Thread Starter

JustMe234

Joined Feb 25, 2017
68
Thank you all for suggestions. I checked all the wiring and all looks tight. The power meter has a similar MCB 25A single pole and when i did the short test in garage even that doesn't trip.

This situation made me paranoid and i give it a try on house. Improvise a short circuit and the MCB on house trip immediately as i short the wires.

From this point i will try to test the RCD with a 5.6 K Ohm as mentioned at post#13 and if it doesn't trip i will replace them all.

One thought is that in a single breaker panel, such as most residential ones, the only portion between the main breaker and the branch breakers is that section of busbar down the center, so that it is unlikely that the main would ever be tripped except by a lightning hit or similar event. And lightning does have enough capacity to force a whole lot of current to flow through even #6 wire for long enough to trip the breaker.
When lightning strike the RCD in the house trip every time.
 

Reloadron

Joined Jan 15, 2015
5,916
The first image has a push to test button. What happens? I agree with KISS in that I am seeing a GFCI and not a circuit breaker in the images. Look up the part numbers. I also see an I (Current) with a Delta sign followed by 30 mA and I believe most GFCIs in Euro are 30 mA? Anyway to me it implies a 30 mA delta between lines through it.

Ron
 
Last edited:

andrewmm

Joined Feb 25, 2011
1,241
Check please, as to do you have RCD or MCB
An RCD, Residual Current , trips when there is a difference between the live and neutral current flowing through it,
i.e. current is going some where else, such as through you,

It will not trip with a short between N and L as the currents are balanced.
he tripping current is typically of the order of 30mA,
The RCD might have a ratting of 100A or above, but that's the maximum current they can interrupt,

If it does not trip with a to ground current, then you need to check the wiring,

An MCB, Max current I think it stands for , trips when an over current is passed through it. These should trip is you short the live to negative,
but can suffer a welding of contacts if they have been tripped a few times in quick succession ( the spark heats the contacts to melting )
 

MisterBill2

Joined Jan 23, 2018
8,446
Upon revisiting that first image, that device shown is not a circuit breaker at all. It is a Ground Fault Circuit Interrupter, (GFCI) with a 40 amp contact rating, and a 30ma trip specification.
THAT means that it provides no short circuit protection what so ever! It should trip if there is 30 milliamps of leakage current from the line to ground, which would be a nasty and painful shock, but not with a 100amp current from line to neutral. So that device is not a mains circuit breaker.
So for short circuit protection in the mains I suggest a fuse pair. My reason for fuses instead of breakers is that when you pull a fuse out there is no chance that the circuit is still live, while a breaker may possibly suffer from welded contacts or some rare mechanical failure and leave the circuit live. Also, you can take the fuses and holder with you, which makes it much harder for others to unknowingly apply power.
 
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