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My understanding is circuit breaker only need to trip when the hot end current jumps up (e.g. short circuit), but I have seen circuit breaker has dual (two) poles with both red cable and blue cable going in and out - what is the reason of connecting neutral to circuit breaker? Also I do not see anywhere around the circuit breaker where there are cables connected to the ground - without ground, how could any voltage have a reference?

Sorry for the newbie question, but I have read a few on circuit breaker but did not find the answer.

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  • \$\begingroup\$ It's probably an RCD. \$\endgroup\$ – Andy aka Mar 15 '17 at 20:00
  • \$\begingroup\$ The RCD compares current in the neutral and hot wire. If they are unequal, the breaker trips. The idea is that if the currents are not equal, there must be some kind of ground fault. \$\endgroup\$ – mkeith Apr 30 '17 at 18:27
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Its not about voltage it's about current. Old style circuit breakers toggle off when the current going through them is enough to trip a relay and work like a fuse.

"Earth leakage circuit breakers" also watch the neutral line. Theory being what ever go's UP must come Down. That is the current going out of the live wire does not match the current returning on the neutral wire, there must be a leak so ground somewhere... However it's a bit of a misnomer, the current could actually 'LEAK' back to the panel through some other neutral path too if the wiring is not correct.

enter image description here

The newest standard also detects electrical arcing in the system.

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    \$\begingroup\$ In the US these are called GFI or GFCI (ground fault circuit interrupt) breakers. I believe that in the UK they are called RCD (residual current device) breakers. In case the OP should wish to use google to learn more about them. \$\endgroup\$ – mkeith Apr 30 '17 at 18:24
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    \$\begingroup\$ Worth noting that here in the UK RCDs and MCBs (i.e. current-sensing breakers for a single circuit) are frequently being replaced with a combined unit called an RCBO ("residual current breaker with overcurrent protection") in new installations. These are more expensive than the old-style boards that use 1 or 2 RCDs that are shared between multiple MCBs, but provide individual protection to each circuit which is more convenient when something does get tripped. These look more like standard breakers than RCDs (which are usually double width), but have extra connections above the regular ones. \$\endgroup\$ – Jules Aug 31 '17 at 4:13
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(Assuming 120V split phase wiring)

The second wire is not neutral, but an additional "hot" wire that is 180deg out of phase with the red. This will appear as 240V at the outlet.

Illustration of split-phase circuit panel

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  • \$\begingroup\$ Data point only: In my country (NZ), breakers for caravans,mobile homes etc require a true two pole P & N breaker with current imbalance tripping as well as current overload. \$\endgroup\$ – Russell McMahon Mar 16 '17 at 4:11
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Dual pole circuit breakers are used in biphasic circuits. In some distribution circuits the demands are supplied using two phases and a neutral cable instead of one phase and one neutral cable. This has advantages for both utilities (it is easier to balance loads) and end-users (sometimes when a fault occurs only one phase ends damaged and the other keeps working). There are also some devices that require high amount of power and are designed to be fed with two phase cables. The red and blue cables you mentioned are probably the two phases of the circuit, not a phase and the neutral (neutral cables are typically white). The circuit breaker does not need connection to any reference, as it acts only as a switch. For that reason there is no ground nor neutral cable connected to it. Finally, you yourself said it "circuit breaker only need to trip when the hot end current jumps up", the breaker trips with high current, not high voltage.

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In your observation, are you assuming the blue wire is Neutral? Seeing that you are in the US, that would NOT be the case. Neutral wires are required to be White in North America. But elsewhere in the world, blue is used as Neutral. Blue conductors here generally are Hot wires from a different phase or pole, depending on your system feed.

Different rules in different parts of the world dictate whether or not you switch the neutral circuit or not. Here in the North America, we do NOT switch the neutral wire for the most part. You CAN if you want to as long as you follow other rules about doing so but in general, that is almost never done. In countries conforming to IEC wiring and grounding conventions, there are different types of power systems that REQUIRE you to switch the neutral conductors along with the "hot" conductors in a circuit. So in that case, the answer would be "in depends".

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In my country (New Zealand), breakers for caravans, mobile homes etc require a true two pole Phase & Neutral breaker with current imbalance tripping as well as current overload.

This makes more sense in many cases as it ensures that both connections are broken regardless of how the input is wired.It makes especial sense in the case of equipment fed via flexible leads or temporary circuits which have a much higher risk of having connections swapped than do fixed wiring.

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Here's a French example...

enter image description here

Large breaker on the left is the RCD which feeds standard breakers. They cut both live (red) and neutral (blue) for extra safety. Normally touching the neutral should be safe, but there are corner cases where it isn't... so it is mandatory to cut both, at least in new installations. Note this is not two-phase (120V+120V=240V) like in North America, this one is just one phase and neutral.

Protection Earth is never interrupted, obviously.

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