# MCB Does not Trip on short circuit.

#### LesJones

Joined Jan 8, 2017
3,139
The OP was concerned about devices with very thin cables being protected if there was a short at the appliance end of the thin cable which is why I suggested a 6 amp MCB. (Which is the lowest rating I have seen in the UK.) Living in the UK I don't know how this situation is dealt with in the many EU countries. The sockets I have seen in EU countries I have visited have been the 16 Amp Schuko sockets. So if the MCB (Or fuse) is rated at 16 amps that feeds the socket then for the appliance cable to be protected it must be rated for 16 amps even if the appliance only takes a very small current. In the UK most mains sockets are the 13 amp type. The cable to the sockets is protected by the MCB but all 13 amp plugs have a replaceable fuse fitted. (These are available in 2, 3, 5, 7, 10 and 13 amp ratings.) So the appliance cable can be protected by a suitable rated fuse in the plug. It is not foolproof as the user can fit a higher rated fuse than the correct one.

Les.

#### Reloadron

Joined Jan 15, 2015
5,916
The terminology is different between the US and UK or Europe for that matter. In the US we are expected to abide by the NEC (National Electric Code). My last NEC book was likely 1,000 pages and was a 2008 version. The NEC is published every 3 years last I knew. The NEC gives in fine detail exactly what is permitted and not permitted, and it is almost trivial to inspect and approve an installation on a point by point analysis, as frequently occurs on this very forum, which is not to say points aren't sometimes contentious. You pretty much go to school to learn how to interpret the NEC. Been there and done that at company expense. While interesting the bar and grill across from the course featured outstanding corned beef sandwiches. I remember the lunches more than the class material.

The UK has their own rule book. BS7671 which I only saw once in my life and as I recall runs about 300 pages. The NEC for example babbles on on which cable to use while the BS standard just says think about your application and needs. The end result in either case is a design which is safe and which will pass an inspection. Naming conventions also differ greatly. I have and install GFCI (Ground Fault Circuit Interrupters) which trip at about 5 to 6 mA while the UK has a RCD (Residential Current Device) which trips at 30 mA. Breakers are MCBs, miniature circuit breakers, bigger breakers are MCCBs (molded case CBs), even bigger are ACBs (Air CBs).

It also does not help when images are posted which have been photoshopped to remove data. Then we are only left with what a thread starter decides to name something. My receptacles in my garage are 120 VAC 20 Amp service with the exception of my 240 VAC 30 Amp service. They are all AWG 12 with the exception of my 30 amp service which is AWG 10. Even if I use a piece of AWG 18 to intentionally short an outlet a 20 amp circuit breaker will disconnect immediately. All of my 120 VAC 20 amp service outlets are GFIC protected as is my garage door opener and all outside lighting service and outside weather protected outlets.

I suggest the thread starter review the applicable code and comply with it.

Ron

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,679
The uk also has ring mains which mess up tradional calculations.

Thread Starter

#### JustMe234

Joined Feb 25, 2017
68
The OP was concerned about devices with very thin cables being protected if there was a short at the appliance end of the thin cable which is why I suggested a 6 amp MCB. (Which is the lowest rating I have seen in the UK.) Living in the UK I don't know how this situation is dealt with in the many EU countries. The sockets I have seen in EU countries I have visited have been the 16 Amp Schuko sockets. So if the MCB (Or fuse) is rated at 16 amps that feeds the socket then for the appliance cable to be protected it must be rated for 16 amps even if the appliance only takes a very small current.
All heavy load appliances i connected to Schuko (16 amps) sockets as you mentioned. The wires that feed those sockets is 2.5mm.
It also does not help when images are posted which have been photoshopped to remove data. Then we are only left with what a thread starter decides to name something.
I suggest the thread starter review the applicable code and comply with it.
Thank you for your explanation.
All necessary information that needed is in those photos. The part that is "photoshopped" is the place where labels are placed, i don't think that will be helpful. As I mentioned early in posts Welder is connected in a 20A MCB also Compressor in another 20A.

#### Reloadron

Joined Jan 15, 2015
5,916
These are MCB types with detail.
Types of MCB based on Tripping Characteristics

MCBs are classified into different types according to tripping over the range of fault current. The important MCB types are as follows:
1. Type B MCB
2. Type C MCB
3. Type D MCB
4. Type K MCB
5. Type Z MCB
The link provides the tripping current and operating time of each of the above MCB types. Things are pretty well explained. Your MCB is either broken or it isn't. Consider the Type and time factors. Placing a dead short across AC mains is not exactly the recommended procedure for testing a circuit breaker. Earlier images reflected a press to test button. Those appear to be what I call a GFCI and in the UK is a RCD (Residential Current Device). I asked what happens when you press the press to test? Regardless if you suspect faulty anything I would get the services of a competent electrician and have things checked and inspected.

Ron

#### MaxHeadRoom

Joined Jul 18, 2013
22,866
The uk also has ring mains which mess up tradional calculations.
I think that is a good system, in general.

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,679
Ron:

Turn the aging clock backwards. Your memory seems to be failing:

RCD (Residential Current Device)
The R for RCD is Residual, nor Residential
Was that a bit, nibble or parity error?

#### MisterBill2

Joined Jan 23, 2018
8,474
These are MCB types with detail.
Types of MCB based on Tripping Characteristics

MCBs are classified into different types according to tripping over the range of fault current. The important MCB types are as follows:
1. Type B MCB
2. Type C MCB
3. Type D MCB
4. Type K MCB
5. Type Z MCB
The link provides the tripping current and operating time of each of the above MCB types. Things are pretty well explained. Your MCB is either broken or it isn't. Consider the Type and time factors. Placing a dead short across AC mains is not exactly the recommended procedure for testing a circuit breaker. Earlier images reflected a press to test button. Those appear to be what I call a GFCI and in the UK is a RCD (Residential Current Device). I asked what happens when you press the press to test? Regardless if you suspect faulty anything I would get the services of a competent electrician and have things checked and inspected.

Ron
It will be very much less expensive to imply replace the suspected part than to engage the services of an electrician. At least in my area the cost is over $100 for them to show up. Parts and labor are in addition to that. Of course, that is presuming that the present installation is correct. #### Alec_t Joined Sep 17, 2013 12,007 UK has a RCD (Residential Current Device) Residential Residual (i.e the difference between line and neutral currents). Edit: KISS beat me to it. Last edited: #### Reloadron Joined Jan 15, 2015 5,916 Ron: Turn the aging clock backwards. Your memory seems to be failing: The R for RCD is Residual, nor Residential Was that a bit, nibble or parity error? It was only a matter of time. Residual in a residence? Well anyway I was well intentioned, not well versed apparently. Residual makes sense. Thanks Ron #### Reloadron Joined Jan 15, 2015 5,916 Residential Residual (i.e the difference between line and neutral currents). No clue why I typed that wrong. Thank you. Duh, Delta as in the delta symbol on the devices. Ron #### MisterBill2 Joined Jan 23, 2018 8,474 Are those Residual Current Difference devices at all similar to the Arc Fault Detector breakers that were all the fad here for a while?? I call that temporary mandate a fad because it was enacted by those who really were a bit detached from reality. After-the-fact it was found that many of the better switcher power supplies would trip them repeatedly. And how many problems are actually caused by arcing that does not trip a breaker? #### MaxHeadRoom Joined Jul 18, 2013 22,866 Are those Residual Current Difference devices at all similar to the Arc Fault Detector breakers that were all the fad here for a while?? RCD = GFI. IIRC they came into general use in the UK around the late '50's ! Mainly due to the introduction of non-metalic water supplies to Residences. Max. #### Reloadron Joined Jan 15, 2015 5,916 Are those Residual Current Difference devices at all similar to the Arc Fault Detector breakers that were all the fad here for a while?? I call that temporary mandate a fad because it was enacted by those who really were a bit detached from reality. After-the-fact it was found that many of the better switcher power supplies would trip them repeatedly. And how many problems are actually caused by arcing that does not trip a breaker? No a GFCI and an AFCI are different animals. The NEC initially mandated arc fault detectors in all new construction but the NEC is merely a guideline. Just as an example: " 210.12(C) is a new Section in the 2014 NEC that requires AFCI protection for the branch circuits supplying 125V, 15A or 20A outlets in dormitory bedrooms, living rooms, hallways, closets and similar rooms. The AFCI protection must be one of the means described in 210.12(A) (1) through (6)". This was initially shot down in many states and the main NEC target was new construction. The problem was contractors. Most new construction involves 200 Amp service @ 240 Volts. The breaker panels can contain 40 circuits or more. The average AFCI / GFCI breaker cost about$40 USD plus. Initially the NEC wanted all breakers to be AFCI and GFCI which really added unnecessarily to the construction cost. State laws for new construction trump the NEC. While many states simply use the NEC and asopt it they can and do call out exclusions.

Even GFCI has it's place. Motors, especially older motors as used on floor buffers or large mixing machines have both capacitive and inductive leakage to ground. The GFCI breakers trip and I have seen it extensively on older refrigeration compressors. Like anything there is a time and place for GFCI and AFCI but not every time and place.

Ron

#### MisterBill2

Joined Jan 23, 2018
8,474
OK, and thanks. The worst GFCI caused disaster I have seen was an instance when a heater tape installed on outside water pipes was shut off because some snow melted on the plug into the extension cord. So the pipe froze and cracked open. But of course a potential shock hazard was prevented. Since it was oldered copper piping the repair was neither simple nor cheap. One rather unhappy homeowner.

#### anniel747

Joined Oct 18, 2020
612
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.
The fuse was added because of the chinese APC recall some time ago. They would catch fire if there was a surge.