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I have two questions. I couldn't find it in google or archives.

First. When you accidentally touch both line and neutral or line to line with your fingers or different parts of your body (for example, fingers to line, toe to neutral or another line), what is the minimum amperage of the circuit breaker before it can trip? For example, if you have a 60 ampere circuit breaker.. would it trip when you accidentally touch line and neutral, or line to line?

Second. If line and neutral touch themselves, what is the maximum breaker amperage when it can still trip. Like if the breaker rating is 500 Ampere.. it may no longer trip when the line and neutral touch would it. I want to know if specifically 60 Ampere breaker can trip if line to neutral or line to line shorts or touches.

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In general, circuit breakers provide protection from overloads and short circuits. Their primary purpose is to protect the wiring - overloaded wiring will overheat, break down insulation, and eventually start a fire.

They are designed to trip immediately on short circuit, when you have line to line or line to neutral contact. With a short circuit, you have extremely high current.

They are not designed to trip immediately on overload at their nominal protection rating; in other words, your 60 amp breaker won't trip immediately under a 61 amp load. There is a "trip curve" for the breaker, which describes how long it takes to trip on overload - longer / slower for a slight overload, shorter / faster for a more dramatic overload.

When accidental contact causes current to flow through your body, your body's resistance is high enough that the current is usually quite small; it only takes tiny currents to cause injury or death. This won't trip a regular breaker - it isn't a short circuit or an overload. Regular circuit breakers are not designed to trip for safety.

Note that faults that travel through ground are also usually through high enough ground resistance that small currents flow, not enough to trip a breaker. When an equipment grounding system is in place, it will provide a low resistance path for many faults, allowing enough current to flow to quickly trip the breaker and "clear the fault."

GFCI / GFI / RCD circuit breakers have another means of tripping. A GFCI device monitors for the current on each of the two or three wires to see if there's any current going out on one wire that isn't returning on the others, which means there must be current flowing through ground, which could be a person receiving a shock. There are different types of GFCI, but common US class A GFCI trips at only 4-6 milliamps of current, low enough to prevent injury. Other types have higher trip levels.

Note that a GFCI only protects against accidental contact that causes a ground fault; under some circumstances you can still receive a shock from a GFCI. For example, if you are wearing rubber soled shoes on a dry wooden floor and are making no ground contact, and you grabbed a bare neutral with one hand then a bare line conductor with the other, there would likely be no current flowing through the ground, and the GFCI would not trip.

  • I bought a 60 Ampere GFCI 2-pole breaker just to power an oven, and I couldn't return it. I should have bought the 30 Ampere version. I was supposed to connect it in series with existing 30 Ampere normal breaker.. but because I couldn't connect the plug in type without a panel.. and I ordered a 6 branches plug-in panel. I can no longer connect it to the existing 30 Ampere in series but 100 Ampere main. So it's only good for short circuit and GFCI function. But notice even if your load is an oven, your breaker in panel has rating of 20 Ampere which is still over size.. is my 60 Ampere so high? – Samzun Nov 5 '18 at 10:43
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    @samzun - I am not quite following but this sounds like way more than a comment, why not post another question? – batsplatsterson Nov 5 '18 at 10:49
  • It would be too complicated... just want to know whether you can use 60 Ampere breaker just to power an oven? So the protection is only for short circuit and not overload.. isn't it... – Samzun Nov 5 '18 at 10:54
  • btw.. this is related to your answer.. why is there a warning in my Siemens gfci breaker in the Safety Instructions part that: "To obtain maximum protection against electric shock, electric ranges and clothes dryers whose frames are grounded by connection to the grounded circuit conductor should not be connected to the load circuit of this device." Why? – Samzun Nov 5 '18 at 10:59
  • @samzun, if I follow, I believe that is referring to the practice of bonding the neutral to the frame of the appliance when connecting these appliances with a three wire cord with no equipment grounding conductor. Connected that way, the frame of the device can return ground current through the neutral, which would hide the imbalance from the GFCI, preventing it from tripping on a ground fault that creates a high impedance ground fault. (A dead short / low impedance fault would trip the breaker on overload / short, not GFCI.) – batsplatsterson Nov 5 '18 at 11:13
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A breaker will trip when the current through it exceeds its rating. (With a few caveats with regards on how long it takes them to trip).

Breakers also have a voltage rating which tells how high of a voltage they can break reliably. If you exceed that rating the current may continue to arc between the contacts.

However it only takes a few milliAmps to kill you. This is way below what breakers will trip at.

So you cannot rely on them to keep you alive. Their main purpose is to prevent fires from wires overheating and igniting the structure.

  • But in many homes.. if your load is only 5A or 9A or 12.. most use uniform 20A breaker.. not good? Or consider most of your load off at night and only your 2 Ampere CCTV opened.. the 20A breaker would be 10 times the load. What do you make of this common scenario? – Samzun Nov 5 '18 at 12:27
  • The breaker rating is the maximum current it will allow through before it breaks. It is perfectly happy letting through less indefinitely. – ratchet freak Nov 5 '18 at 12:35
  • In that case, the use of the oversized breaker is for tripping direct shorts between line to neutral.. or line to ground. I'm just justifying the purchase of 60A breaker instead of 30A. Do you know the maximum amperage of a circuit breaker when it would no longer trip? like 200A for shorted line to neutral? – Samzun Nov 5 '18 at 12:43
  • In a correct circuit breakers will always trip in the presence of a short. The max rating of a breaker is decided by the wires it is protecting. Overrating a breaker risks fires when the wires are overloaded. – ratchet freak Nov 5 '18 at 12:58
  • @Ratchet freak home breakers do not trip at there rated value for an overload in fact they may Cary 3x to 5x there rated value and as blatsplaterson explained trip curves for standard breakers are called inverse time breakers if a short the trip or open quickly but if the load on a 60 amp breaker is not a short but for example a motor load the breaker will hold for 10-20 seconds at 120 amps or higher based on the trip curve for that breaker. There are also rules for the maximum size of breakers usually 150% for corded devices a 3 amp load would be listed for 15 amp circuit but can use 20a +BP. – Ed Beal Nov 5 '18 at 14:56
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If it's a US GFCI breaker, it'll trip within 20ms on excess of 8 milliamps, which may be a memorable experience for you.

A Euro RCD (=GFCI) breaker will trip on excess of 30ma, which can stun and might kill you.

A Euro RCD rated for human protection will trip at around 6ma.

All other breakers, depends who you are. If you are Colossus from X-Men, you are made of metal and it will trip the breaker. Deadpool or Claire from Heroes, it will hurt until you let go, if you even can let go. If you are Bruce Banner, it will make you very angry. Thanos, anyone's guess. Everyone else, you are dead. Sorry, Jean Grey. The person who finds your body is also dead.

  • Harper, in the thread "Too small ground wire". You stated "The thermal trip is sized to warm at about the same speed as the wire in the walls, so it trips somewhat before the wire warms enough to be dangerous. That requires matching between breaker and wire size.". What if the wire in the wall is put near ice. Then it won't warm enough.. would this trip the breaker? This confused me for a month. It's as if the breaking tripping depends on the temperture in the wire. Couldn't it be easier to say it trips on current at breaker? – Samzun Nov 6 '18 at 12:55
  • @samzun it's the best way to make assumptions about the breaker vs. wire temperature. They don't actually embed thermometers in the wire and have them communicate with the breaker. You have contrived an edge condition that reduces the method's relevance, but if you go to all that trouble to chill the cable, then yeah, you get to uprate the cable whilst the refrigeration plant is in operation. I believe there's a derate/uprate table for that in NEC. – Harper - Reinstate Monica Nov 6 '18 at 17:50

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