A circuit breaker is designed so that it will trip when the electric current is too high. That is the purpose of a circuit breaker. If there is a metal piece on the circuit breaker that prevents it from tripping, it is useless. Perhaps someone has altered it.
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To protect a circuit, we use a fuse or circuit breaker. The fuse or circuit breaker must always be placed in the lineconductor, never in the neutral conductor. So, if we want to completely isolate a circuit from the supply, we must remove the fuse from the line conductor, and open the link in the neutral conductor. A 'link' is a non-fusible metal break point in the neutral.
NO! NO! NO! The circuit breaker must be rated for the line voltage used. Exceeding this limit can cause catastrophic failure! If you are really working on a 1600A bus, stop right now, ang go hire a professional! Your question indicates you are not qualified to work on this gear. YOU ARE GOING TO KILL SOMEBODY. I'm not kidding about this. A breaker that is applied beyond its rating can arc over, and the resulting arc-flash can punch right through the panel cover, spraying vaporized metal on anyone unfortunate enough to be standing there. A 1600A bus can have a fault current of 20,000A OR MORE. This is not something to even think about working on unless you are trained and absolutely know what you are doing!
Devices used to open an electrical circuit are called OCPD's. (overcurrent protection devices) The most common are circuit breakers and fuses. In commercial motor starters they often employ another OCPD called a "heater" (UK thermal fuse) which is a metal strip which melts like the element in a small fuse when too much current (amps) go through it. They devices are in the circuit primarily to protect the wiring and devices they power. Circuit breakers such as in your home panel do protect you from a "shorted" circuit. Another safety and most common in residences are GFCI's or ground fault circuit interrupters (UK ELCB, Earth Leakage Circuit Breaker or RCD, Residual Current Detector) which come as both a receptacle (female plug) or as a circuit breaker.
The bare wire is the equipment grounding conductor. Its purpose is to ground the metal parts of equipment that are not part of the circuit. This assures the proper function of the breaker in the event of a fault. It exists for your safety and disregarding it exposes you to potential danger, even death.
Any such attempt with a meter would just destroy the meter. Why not just find out which breaker/fuse controls the circuit and see what the breaker/fuse is rated at. Also find out what the wire size is. For a 20 AMP circuit you MUST have 12 gauge wire. There are other requirements but those two seem to be about the most significant. As has been stated, a meter won't help. The breaker prevents excessive load to be placed on the wiring so any 20 amp circuit would be limited by the breaker and there would be no other way to determine if a circuit is 15 Amp or 20 Amp other than to check at the breaker panel. Also, if someone just put a 20 amp breaker on a 14 gauge wire you have a serious fire hazard. As the above post suggests, have a professional check this out for you, it's too dangerous to make assumptions. Your description of "20A dedicated line" makes me think that you are needing a circuit for a heavy power-using-device that needs it own circuit (without any other receptacles or devices). If that is the case, then a meter can be used (with difficulty) to determine if any other outlets are connected to the same circuit. First turn off the power on the receptacle. Then check to see if any other receptacles are also dead. If so, they are probably on the same circuit and your receptacle is not a dedicated supply. Another method is to use a circuit tracer (available at Lowe's and other hardware and electrical supply stores). Plug the transmitter into the questionable receptacle, then use the receiver to check other receptacles to see if they are on the same circuit. Receptacles on the same circuit will cause a continuous fast beep (or other signal depending on model). You can also trace the circuit to the circuit breaker panel to find which breaker it is connected. It should be a 20-Ampere-trip breaker to make full use of a dedicated 20 A receptacle. Borrow an amp clamp and find the breaker in question. Clamp it around the neutral for that circuit (This takes a friend helping). Plug in a significant load like a hair dryer and have the friend turn in on and off while you watch the meter. It should read zero, then 8-10 amps then zero, etc. Plug loads in outlets that you think the contractor absconded the circuit from. The amps from those outlets should never show up on the amp clamp. Make sure the wire is 12 awg. A 12 awg wire will not push in the stabs of a 15 amp receptacle. SAFETY ADVICEBefore you do any work yourself,on household electricity supplies, equipment or appliances,always turn off the powerat the breaker box/fuse panel BEFORE you attempt to do any work ANDalways use an electrician's test meter having metal-tipped probes(not a simple proximity voltage indicator)to ensure the circuit is, in fact, de-energized. IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOBSAFELY AND COMPETENTLYREFER THIS WORK TO QUALIFIED PROFESSIONALS.