FSP0009 – Electrical Safety Devices – Facility Science Podcast #9

By | June 25, 2019

 

Notes for FSP0009 – Electrical Safety Devices
This is about electrical safety devices, specifically circuit breakers, fuses, GFCI (RCD) and AFCI (or AFDD).
Two types of overcurrent devices with similar function: circuit breakers and fuses
Circuit breakers
  • Opens the circuit when more than the rated current is detected.
  • In the context of power distribution in buildings, a circuit breaker is most often a switch that can be manually operated to turn power to a circuit on and off, and will also disconnect power automatically when excessive current passes through. Some can even be electrically operated by some remote control mechanism.
    • Many circuit breakers (maybe all of them, practically) have an indicator to notify the operator that the breaker tripped off due to a fault condition rather than was turned off manually. In some cases the tripped condition is indicated by a third switch position (so there’s an on position, an off position, and a tripped position…usually you have to move the switch from the tripped position to the off position before returning it to the on position). In other cases, the trip indicator is a second mechanism that provides visual indication of a trip and provides some way to clear the trip and reset the breaker.
  • Meant to protect against 2 different, but ultimately related fault conditions, overload and shirt circuit
    • Overload – the high current is caused by attaching too many devices or devices that draw too much current to a circuit.
    • Short circuit – a short circuit condition is when a circuit is completed by an unintended path. In practice this means that a connection has been made directly from the line conductor to either the neutral or ground conductor.
      • Can happen because of a wire coming loose inside of an appliance, a screw being driven into a conduit
      • Since these conductors are designed to carry current to the load (appliance or light fixture or whatever) without consuming much energy, a direct connection between between the line conductor and either the line or neutral will have a very low resistance which means there will be a very high current. As I stated before, very high current is dangerous because it heats up the wires to the point that they can start a fire. Devices and raceways are often designed and installed so that, if a live wire comes loose inside, it will contact something that is effectively guaranteed to create a short circuit. For example. if you have an appliance with a metal case, like a clothes washing machine, the metal case will be bonded to the ground conductor . The ground conductor doesn’t carry current in a normally operating circuit and is there essentially as a safety to carry fault current and create a deliberate overload condition rather than creating a dangerous situation. We want this to happen so that if an exposed conducting part becomes energized (the metal case of our washing machine, for example), the power will be immediately disconnected rather than create a shock hazard for anyone that might touch the appliance.
  • Two mechanisms used to detect high current and break the circuit, temperature and magnetism
    • Temperature – Electrons flowing through a wire cause the wire to heat up. Some breakers use something like a bimetal strip (a device made of two different metals stacked together). The two metals have different expansion rates so changing the temperature of the device causes it to bend. Device like this can be put in the path of current and since current causes heating, the devices can designed to bend away from the conducting path or bend into a disconnecting mechanism when too much current flows.
    • Magnetism – Current flowing through a wire creates a magnetic field around a wire. When you coil the wire, the magnetic field around each turn stacks with the others so that the strength of the magnetic field is proportional to the number of turns. Strength of the magnetic field is also proportional to the current. If you put an object that is affected by magnetism (such as a piece if steel) inside the coil, you can move the object by turning on the magnet (energizing the coil). This kind of device (a device that converts electrical energy into mechanical work in the way is called a solenoid. The solenoid can push (or pull) against a mechanism in such a way that when the current through the solenoid coil gets high enough the force created by the magnetic field is enough to push (or pull) the circuit breaker mechanism open
  • Large majority of circuit breakers are in powder distribution equipment, (distribution panels and switchboards and etc). However there are also circuit breakers in many control panels and appliances.
  • In general it’s a good idea to identify the problem that caused the breaker to trip before turning the power back on.
Fuses
  • Similarly to circuit breakers, fuses also protects against overcurrent conditions (caused by overload and shirt circuit). A fuse is basically conductor (a piece of wire, basically) that is specially sized and shaped to carry electric current as part of a circuit under normal conditions, but to melt and break, opening the circuit when too much current flows.
  • Fuses come in all shapes and sizes from tiny fuses that can fit on the tip of your finger to huge fuses used for power distribution.
Fuses and Circuit Breakers
  • Always use a fuse or circuit breaker that is designed for the particular application.
    • The device should break the circuit under a fault condition, but not break the current under condition that isn’t a fault. You might think, that this should be simple….if the current goes over the rated value, open the circuit, but it isn’t this simple.
      • Some devices, like motors for example, pull a very high amount (often much higher than the rated amount) of current for a short time when they start up, and also they can tolerate excessive current for a short time without being damaged. For this type of devices, you want an overcurrent device (a breaker or a fuse) that will tolerate a certain amount of excess current for a short time before opening the circuit (with the definition of “short time” depending on the actual load in question)..
      • Other devices, like electronics are very sensitive to excess current because their very small parts are easily damaged. For these devices, we want to to open the circuit as quickly as possible once the current has been reached.
    • The device should be rated to handle the voltage and also the available current. This has to do with arcing. Any time you open an electrical circuit in which current is flowing, an arc will form at the point of the disconnect. Disconnecting devices, including circuit breakers and fuses, have to have some means to contain and extinguish any potential arc that could form. The distance over which an arc will form has mostly do the voltage present, and the ability of an arc to sustain itself and potentially expand (say over a longer distance that was originally required to start the arc) has mostly to do with the available current (so basically the power supply and upstream overcurrent protection devices).
      • Arcing in this context is when electric current travels through the air which creates a very high amount of heat. We generally think of air as an insulator (as in it doesn’t conduct electricity), but for any given voltage there is a distance through air over which current will flow. The higher the voltage the longer the distance that can be jumped. The most spectacular example of arcing is lightning. A lightning arc (that’s a bolt of lightning) can be several miles long. That long of an arc through air requires millions of volts of potential difference between points (say the cloud and the ground). For the voltages commonly found inside our buildings (240V or 120V), the arcing distance is very small, some fraction of a millimeter. You might notice that sometimes when you plug something into a wall receptacle you see a little spark. That spark is another example of arcing.
      • I’ll use a fuse as an example. When the wire in a fuse melts, it creates an gap where there is no conducting material. The size of the gap has to be big enough to prevent a sustained arc from forming. This generally means that a fuse for a higher voltage has to create a bigger gap than a fuse for a lower voltage. The fuse also has to have means of extinguishing whatever arc does form. In some fuses this can be accomplished by creating a large enough gap fast enough that the arc won’t have time to become established. Some fuses for higher power situations are filled with other materials (such as sand) to control arcing. Circuit breakers use similar mechanisms to prevent arcing and to extinguish arcs
Resettable Fuses – There is also a thing called a resettable fuse, some of which are even self-resetting. These are typically found inside electronic devices and protect the device from damage due to overload or short circuit. Typically clearing the fault condition and allowing the device to cool will reset the self-resetting fuse. These are typically very simple devices and are more like the circuit breakers I was describing before than the fuses. I don’t know for sure, but I imagine these things are called fuses instead of circuit breakers because they are used inside electronic devices where we would historically have used fuses and the people who developed them for that purpose didn’t think it necessary to use a different name for a device that accomplishes the same purpose.
GFCI (Ground Fault Circuit Interrupter or RCD (Residual Current Device)
  • We call these GFCI or GFI in the US, so that will be my default terminology.
  • In your home, you probably have an electrical outlet in your bathroom or kitchen with two buttons on it (they say something like “test” and “reset”). Maybe you have a portable appliance (like a hair dryer or a fan) and that device has a big plug with two similar buttons on it. That device with the 2 buttons is a GFCI. GFCI are also commonly built into normal circuit breakers and installed into distribution panels.
  • When you touch a live wire in the typical configuration found in your home, current will flow through your body. Your body isn’t a very good conductor, so it won’t be a lot of current. Most likely it won’t be nearly enough current to bother the 15 or 20A circuit breaker protecting that circuit. Unfortunately, 100mA (1/10 Amp) can stop a  person’s heart, which obviously the person won’t survive if they don’t get immediate help. The GFI help protect against this type of danger.
  • How? In a normally operating circuit, in your house, the device is is connected to two wires at the wall receptacle, the hot wire and the neutral wire. If everything is operating properly, any current going into the device from the hot wire should leave the device on the neutral wire. If the current on those 2 wires are different, that means current is flowing from the device to somewhere else which should never happen in a properly functioning circuit. The GFCI device monitors the hot wire and the neutral wire and disconnects the circuit very quickly (25-40ms) if even a small difference (typically 5-30 mA) is detected. This condition of current flowing outside the intended circuit is called a ground fault. These devices are designed to open the circuit for a very low ground fault current (typically lower than would be harmful) and also faster than would be harmful even for a higher fault current.
  • In addition to protecting against electrocution, GFCI can help protect against fires and equipment damage in ways that normal circuit breakers can’t (normal circuit breaker = overcurrent protection device).
  • These are required, mostly in residential places and in places where there is a higher chance for a accidental ground path to occur. Mostly that means places where people may be working near electrical devices and also might have a higher than normal chance to get wet or be interacting with water (bathrooms, kitchens, swimming pools, garages, patios, etc).
AFCI (Arc fault circuit interrupter)
  • Detects a type of fault not detected by a circuit breaker or a GFCI called an arcing fault.
  • Arc faults typically happen between conductors in a raceway or a cable where the insulation has been damaged. This is one of the reasons many safety regulations forbid the use of extension cords in certain situations and why your fire inspector might tell you to remove an extension cord running across the floor or under a door. Impact or abrasion of the cord can cause the insulation between the wires to wear or break allowing the conductors to come close enough together to create arcing between them. This is usually an intermittent condition that typically gets worse over time. Sometimes the result is a failure of the cable or device, but sometimes the very hot arc comes into contact with something flammable and starts a fire. Arcing faults can also occur inside worn, abused, or poorly made appliances and also where cables are spliced together in junction boxes or at switch boxes or receptacles.
  • We classify 3 basic types of arc faults, serial arc faults, parallel arc faults and ground arc faults.
    • A serial arc fault is when arcing occurs between two parts of the same conductor. For example if you splice the hot wire in a junction box, you have connected two wires together with some type of connecting device (a wire nut or something). If the connection isn’t made properly and the two wire can separate slightly inside the splice, you will get an arcing fault in the splice. The same thing can happen inside a cord (for example that extension cord running across the floor). People stepping on the cord could break the conductor inside the insulation causing arcing to occur between 2 pieces of the same conductor
    • A parallel arcing fault occurs between the hot and neutral (or between two phases of a multi-phase circuit).
    • A ground arc fault occurs between the hot wire or the neutral wire and ground. A GFCI will handle ground arc faults, but not the other 2.
  • Arcing is normal in many situations and electrical devices, so AFCI devices attempt to open the circuit only on abnormal or potentially dangerous arcing. They attempt to do this by analyzing the alternating current waveform and looking for the characteristics of arc faults. AFCI proponents point to the potential (and actual) life-saving benefits of detecting and correcting arc faults before someone gets hurt. Many other people, including many electricians,complain that AFCI devices are too sensitive and that nuisance trips are too common to make the devices worth relying on them for life safety (people either replace them with non AFCI devices or learn to ignore them and just reset them without looking for a problem). I can’t tell who’s right, so while you should probably install them where required by regulation, otherwise you’ll have to make up your own mind on their utility.
  • Because of the statistics about how many people are killed or hurt in fires caused by electrical faults, AFCI devices are required in many places (mostly in the US, I think), especially in certain residential areas where people are more likely to be harmed by an electrical fire.

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