Selective coordination is the selection of fuses and circuit breakers in such a way that an overload or short circuit on one branch circuit will cause only the fuse or circuit breaker feeding that circuit to open, without causing upstream circuit breakers or fuses to open. The National Electrical Code (NEC) requires it for emergency circuits and legally required standby systems, where it can prevent blackouts in places like hospitals and elevators where losing power could be hazardous.
But it can be useful in any area of a manufacturing plant, because it limits the extent of equipment shutdown and aids troubleshooting, helping to avoid unnecessary downtime and scrap. What’s more, selective coordination requires fast-acting circuit protection, which can reduce the severity of an arc-flash, making the workplace safer for maintenance and electrical workers.
How Selective Coordination Is Achieved
Setting up selective coordination properly requires understanding the opening characteristics of the overcurrent protective devices as much as their current ratings. A common rule of thumb is to have a 2:1 ratio in the current ratings between upstream and downstream circuit protection devices. For example, if an upstream device is rated at 200 A and a downstream device is rated at 100 A, one would expect the downstream device to open before the upstream one, and that’s what will happen under most overload conditions. However, during a short circuit, the upstream device may open at the same time or before the downstream device. Most circuit breakers have a range of trip times, and these times usually overlap each other at extremely high currents; during a fault condition it’s impossible to be sure that the lower-rated breaker will trip before the higher-rated breaker begins to open.
The use of current-limiting breakers or fuses can help prevent this problem by reducing the current flow during a short circuit. Such a device can prevent a short circuit on one branch circuit from causing a breaker or fuse upstream from opening and interrupting power to other branch circuits.
The same current-limiting devices that aid in selective coordination can also provide major benefits in two other areas: arc-flash hazards and short circuit current ratings (SCCR).
How Current Limitation Helps with Arc-flash
An arc-flash is an unexpected sudden release of light and heat produced by electricity traveling through air, usually caused by accidental contact between live conductors. Arc-flash temperatures can reach 35,000°F, more than three times hotter than the surface of the sun. The incident thermal energy (heat loading) on a person standing in the arc -flash boundary can exceed 120 cal/cm2, which exceeds the capability of most personal protective equipment (PPE). An arc-flash can generate blast pressures as great as 2,000 psi, and it can throw metal fragments at a velocity of 700 mph (the muzzle velocity of a .380 automatic pistol).
The amount of incident energy a worker may be exposed to during an arc-flash is directly proportional to the total clearing ampere-squared seconds (I²t) of the overcurrent protective device during the fault. Higher currents and longer exposure time produce greater incident energy. Current-limiting fuses and breakers shorten the duration of an arc-flash event. A difference of 1/2 second of clearing time makes a big difference in the amount of energy released during an arc-flash event; a current-limiting fuse will open in less than 1/2 an ac cycle (8.3 ms), greatly reducing the energy discharged, as shown in Fig. 1. The gray area shows the energy allowed through by a conventional overcurrent protective device, while the green area shows the energy allowed through by a current-limiting device.
There are multiple regulations and standards concerning arc-flash, including OSHA, the NEC, and NFPA 70E. NEC Article 110.16 requires that “Equipment such as switchboards, panelboards, industrial control panels, meter socket enclosures, and motor control centers that are in other than dwelling occupancies and are likely to require examination, adjustment, servicing, or maintenance while energized shall be field marked to warn qualified persons of potential electric arc-flash hazards.” Figure 2 shows an example.
Arc-flash analysis is a multi-step procedure often best done by specialists, and involves developing circuit diagrams, determining available fault current from the utility or generator, calculating maximum available bolted fault currents and minimum self-sustaining arcing current at each location, and checking the clearing times of all overcurrent protective devices.
How Current Limiting Helps with SCCR
The same current-limiting fuses that help provide selective coordination can also improve the SCCR of an industrial control panel. The SCCR of a control panel is defined as the maximum current, at a specified voltage, that the panel can safely withstand for a specified time, or until a circuit protective device safely operates. The SCCR of the panel must be greater than the maximum possible fault current available at its feed point. The panel’s SCCR is not necessarily equal to the interrupting rating of its main overcurrent protective device (fuse or circuit breaker). Instead, it depends on the “weakest link,” or lowest-rated device, within the panel — with one important exception (see below).
NEC Articles 409, 670, 440, and 230 require that control panels and other equipment that control or distribute power to utilization equipment must have their SCCR clearly marked on their labels (Fig. 3).
By minimizing the peak let-through current, current-limiting fuses or circuit breakers limit the stress on the components in the panel and make it possible to increase the rating of the panel to more than the rating of the weakest link. Other methods of establishing the SCCR of industrial control panels may require testing or coordination with the local authority having jurisdiction, but using current-limiting fuses and UL 508A can simplify the approval process.
A Caveat on SCCR
It is important to note that if a panel feeds multiple loads it may be difficult or impossible to achieve selective coordination if it is protected by current-limiting fuses in the feeder circuit. If there is a short circuit on one of the loads it is likely to cause the current-limiting fuse to open and shut down the whole panel. The lesson here is to try to avoid feeding multiple loads from the same feeder circuit, if selective coordination is an issue.
One way to make selective coordination easier to accomplish is to install a so-called coordination panel (Fig. 4). In this type of panel each individual branch has a circuit breaker followed by a current-limiting Class CC or Class J fuse. A normal overload will trip the breaker, just as in a conventional panel, but if there is a short circuit the fuse will open and prevent overcurrent protective devices upstream from opening.