If you have two resettable breakers in line and the first one nearest the appliance is a 16amp breaker and the second one is the main fuse box 30amp breaker, why if the 16amp breaker trips due to a short circuit on the appliance side of the 16amp breaker why does the 30amp breaker also trip. The fault is a short circuit on the appliance side of the 16amp breaker.

Is it something to do with breaker reaction time ratings?

Thanks to all those more knowledgeable.

 

Under "Short-Circuit" conditions,
the Current may spike extremely high for a very short period,
far higher than ~30-Amps.

Your Circuit-Breakers are most likely a combination of Thermal and Magnetic.
The Thermal function is for "over-loads" that are just creeping over the rating of the Breaker.
The Magnetic function is for fast response to a direct-short condition.

There is a very good chance that the Circuit-Breaker near the appliance is "Thermal-only",
but the Circuit-Breaker in the Distribution-Panel is almost certainly a combination-Thermal and Magnetic type.

"Thermal-only" Circuit-Breakers are usually quite slow to respond,
so under a Direct-Short-condition it may allow a huge Current-Spike to flow
which will easily trip the ~30-Amp Magnetic-Breaker.

A Thermal-type-Breaker may never trip at all if there is a Magnetic-Breaker in line before it,
even if the Magnetic-Breaker is substantially higher-rated than the Thermal-Breaker.

Magnetic-Breakers are usually somewhat large and have some definite weight to them,
Thermal-Breakers may be quite cheap and small, sometimes just a tiny Reset-Button.
.
.
.

 

In my first "office" I had a bank of 20A and one main 100A. I designed power supplies and shorts are common. The 100A was fast and the 20A were slow. I never tripped the 20A breakers. Just saying I have been there before.

 

If you have two resettable breakers in line and the first one nearest the appliance is a 16amp breaker and the second one is the main fuse box 30amp breaker, why if the 16amp breaker trips due to a short circuit on the appliance side of the 16amp breaker why does the 30amp breaker also trip. The fault is a short circuit on the appliance side of the 16amp breaker.

Is it something to do with breaker reaction time ratings?

Thanks to all those more knowledgeable.

This is a problem know as "selectivity", and in a correctly designed system the nearest breaker only will trip and leave the rest of the installation functioning. This is of course very important in the design of emergency lighting systems, where it is essential that on faulty sub-circuit doesn't turn off the lights in the entire building. It requires a thorough understanding of the characteristics of each fuse and circuit breaker in the system and the magnitude of fault currents.

 

Under "Short-Circuit" conditions,
the Current may spike extremely high for a very short period,
far higher than ~30-Amps.

Your Circuit-Breakers are most likely a combination of Thermal and Magnetic.
The Thermal function is for "over-loads" that are just creeping over the rating of the Breaker.
The Magnetic function is for fast response to a direct-short condition.

There is a very good chance that the Circuit-Breaker near the appliance is "Thermal-only",
but the Circuit-Breaker in the Distribution-Panel is almost certainly a combination-Thermal and Magnetic type.

"Thermal-only" Circuit-Breakers are usually quite slow to respond,
so under a Direct-Short-condition it may allow a huge Current-Spike to flow
which will easily trip the ~30-Amp Magnetic-Breaker.

A Thermal-type-Breaker may never trip at all if there is a Magnetic-Breaker in line before it,
even if the Magnetic-Breaker is substantially higher-rated than the Thermal-Breaker.

Magnetic-Breakers are usually somewhat large and have some definite weight to them,
Thermal-Breakers may be quite cheap and small, sometimes just a tiny Reset-Button.
.
.
.

LowQCarb

That is so well explained and I have learnt today much more than I anticipated.

My 16amp MCB is a type B but it doesn't say whether Thermal only or Thermal and Magnetic. Hager HAG NBN116A



Many thanks

This is a problem know as "selectivity", and in a correctly designed system the nearest breaker only will trip and leave the rest of the installation functioning. This is of course very important in the design of emergency lighting systems, where it is essential that on faulty sub-circuit doesn't turn off the lights in the entire building. It requires a thorough understanding of the characteristics of each fuse and circuit breaker in the system and the magnitude of fault currents.

Thank you for the info.

 

I'm not familiar with other countries,
but if you're in the USA,
all of the Circuit-Breakers in your home's Breaker-Panel are combination-Thermal and Magnetic-type.
This is a universal requirement of the NEC ( National-Electrical-Code ).

Since You mentioned a "16-Amp" Circuit-Breaker,
You most likely live in a different Country since a "16-Amp" Circuit-Breaker is almost unheard of here.
.
.
.

 

I'm not familiar with other countries,
but if you're in the USA,
all of the Circuit-Breakers in your home's Breaker-Panel are combination-Thermal and Magnetic-type.
This is a universal requirement of the NEC ( National-Electrical-Code ).

Since You mentioned a "16-Amp" Circuit-Breaker,
You most likely live in a different Country since a "16-Amp" Circuit-Breaker is almost unheard of here.
.
.
.

16A breakers are common here, but not 30A. The common size is 32A. (so not Britain either)

 

16A breakers are common here, but not 30A. The common size is 32A. (so not Britain either)

Apologies it is a Hager MTN120 B20 circuit breaker data sheet attached for information. So 20 amp not 16amp as stated earlier. The main distribution board breaker is 32amp not 30amp as stated earlier. Since been and checked. My bad.

 

What type is the circuit breaker in the main distribution?
The MTN120 looks ok (although a 20 A rating is unusual - at least here in Germany) - magnetic+thermal, fast-acting.
The 32 A breaker should better be a type C breaker (someehat slower than type B) - that's called fuse coordination

Another aspect to consider: what else is drawing current from the 32 A CB when the short occurs?
(IOW: what other loads are connected to the 32 A CB?)

 

The **Hager MCB 1P 6kA B-20A 1 M** breaker is a **thermal-magnetic** circuit breaker.

- The **thermal** mechanism protects against **overload** conditions by using a bimetallic strip that bends and trips the circuit when prolonged overcurrent occurs.
- The **magnetic** mechanism provides protection against **short circuits** by using an electromagnetic coil that instantly trips the breaker when a sudden surge of current is detected.

The "B" curve rating indicates that this breaker will trip within 3 to 5 times its rated current (20A) under short-circuit conditions, which is typical for general domestic applications.

The breaker was selected based on the maximum current that would be drawn on the circuit at any one time determined by the following table and being less than the main 32A circuit breaker with the intention of power tool operation never tripping the main fuse board breaker:



Thank you for your continued interest.

 

In understand your considerations - although that's not what a CB is intended to be used for
Circuit breakers protect the lines (from burning)! For 1.5 mm², a 16 A CB is considered appropriate (2.5 mm²: 25 A). So it depends on your installed lines whether the 20 A might pose a risk.
You didn't give away the characteristics of the 32 A CB. If this is a type B and more at the lower end (3 * nom. current) while the 20 A Hager is more at the higher end (5 * nom. current), this would even mathematically explain the phenomenon you're experiencing
But could as well be that the larger CB is trippjng faster (in terms if time to break) than the smaller one. Specification uncertainties and tolerances can be quite tricky (And when it comes to tolerances, fuses and circuit breakers ARE tricky. I once spend weeks qualifying some fuse for a "quite special" application.)

 

In understand your considerations - although that's not what a CB is intended to be used for
Circuit breakers protect the lines (from burning)! For 1.5 mm², a 16 A CB is considered appropriate (2.5 mm²: 25 A). So it depends on your installed lines whether the 20 A might pose a risk.
You didn't give away the characteristics of the 32 A CB. If this is a type B and more at the lower end (3 * nom. current) while the 20 A Hager is more at the higher end (5 * nom. current), this would even mathematically explain the phenomenon you're experiencing
But could as well be that the larger CB is trippjng faster (in terms if time to break) than the smaller one. Specification uncertainties and tolerances can be quite tricky (And when it comes to tolerances, fuses and circuit breakers ARE tricky. I once spend weeks qualifying some fuse for a "quite special" application.)

The 32A main circuit breaker is a CHNT NB1-63 B32 which is both magnetic and thermal.

The main circuit is to provide power socket outlets in a block of 6 garages each of which has a 2 gang outlet and is designed for typical UK domestic installation. Wiring for the socket outlets from the mains 32A circuit breaker is a typical domestic 13-amp ring main comprising 2.5 mm² cross-sectional area (CSA) copper conductor covered by PVC insulation and is capable of handling the current capacity required for standard household sockets. Additionally, a 1.5 mm² CSA is commonly used for the earth wire (CPC - Circuit Protective Conductor) in this type of circuit. The wiring is protected by the 32-amp circuit breaker in compliance with UK wiring regulations (BS 7671).

 

The 32A main circuit breaker is a CHNT NB1-63 B32 which is both magnetic and thermal.

The main circuit is to provide power socket outlets in a block of 6 garages each of which has a 2 gang outlet and is designed for typical UK domestic installation. Wiring for the socket outlets from the mains 32A circuit breaker is a typical domestic 13-amp ring main comprising 2.5 mm² cross-sectional area (CSA) copper conductor covered by PVC insulation and is capable of handling the current capacity required for standard household sockets. Additionally, a 1.5 mm² CSA is commonly used for the earth wire (CPC - Circuit Protective Conductor) in this type of circuit. The wiring is protected by the 32-amp circuit breaker in compliance with UK wiring regulations (BS 7671).

If the ring main supplies all 6 garages and is protected by the 32A MCB, where is the 20A MCB? If you've got a copy of the wiring regs, then the tripping time vs. current for all current capacities of type B, type C and type D MCBs are all in there.

 

Is it a CHINT NB1-63 1P B32 ? (This type is available in 1P, 2P, 3P and 4P variants.)
Have you got it from aliexpress? They are cheap like dirt but maybe "China Export" quality :-(

If so, in your installation 2 type B CBs are competing about which is the faster one
And they may well trip both because once the CB has got a large-enough "triggering pulse", there's nothing that would stop it from breaking the circuit.
This type of CB is also available as C32, which might or might not solve your issue - depending on the "basic load" from the other 5 garages on the main CB.

BTW: "British installation" (" domestic 13-amp ring main") is considered somewhat "funny" here (Continental Europe) although I understand the initial reason (shortage on copper) for this type of installation. And the "earth wire" is really only 1.5 mm² ? Every day I learn something new.
Plus: your wall plugs can easily serve as a weapon

 

Plus: your wall plugs can easily serve as a weapon

The perfect caltrop. Ten times worse than standing on a lego brick. And the cable entry is designed so that any cable lying on the floor will force the pins to point upwards.

At a guess, all the MCBs will be B type because that's what you get if you don't specify anything else. Swapping to a C or D might solve the problem.
And Chint are a good make these days. I use lots of their MCBs and contactors.

 

I've checked CHINT offers on aliexpress, finding only images of Cxx breakers (but nothing - or type C - within the descriptive text). Their RCBs are also quite cheap, so maybe I should get some of these for my next refurbishment project

 

Is it a CHINT NB1-63 1P B32 ? (This type is available in 1P, 2P, 3P and 4P variants.)
Have you got it from aliexpress? They are cheap like dirt but maybe "China Export" quality :-(

If so, in your installation 2 type B CBs are competing about which is the faster one
And they may well trip both because once the CB has got a large-enough "triggering pulse", there's nothing that would stop it from breaking the circuit.
This type of CB is also available as C32, which might or might not solve your issue - depending on the "basic load" from the other 5 garages on the main CB.

BTW: "British installation" (" domestic 13-amp ring main") is considered somewhat "funny" here (Continental Europe) although I understand the initial reason (shortage on copper) for this type of installation. And the "earth wire" is really only 1.5 mm² ? Every day I learn something new.
Plus: your wall plugs can easily serve as a weapon

Its 1 P and was supplied by a well known long establish electrical factor here in the UK. Your point is noted and maybe I will purchase another 32A CB.

Thanks

 

Yes, you’re right! It’s likely due to the breaker reaction times. Both breakers can trip during a short circuit because they detect the same fault, but their tripping speeds can overlap. Even though the 16A breaker is closer, the 30A breaker might react fast enough to trip as well, especially if it has a sensitive response to short circuits.

It’s also possible that the high current during a short circuit is enough to affect both breakers at the same time.

 

The problem is because the other speed rating breakers are seldom available thru common suppliers. So for protection in a shop, getting a third breaker with a lower current rating and a faster trip response would be the logical choice. It might even be that over the years such a design has been posted. The good news is that since the circuit is already "protected", the additional breaker will not require any certification or approval.

 

OK guys thanks very much. I've now solved the tripping problem. It was due a short circuit as a result of a miss wire.