Training electrical workers to see the safety differences between overcurrent protection and overload protection
Fuses and circuit breakers are located at the beginning of most circuits but at the end of a service. Could Art. 230 [Services] be an exception to Art. 240 [Overcurrent Protection], and are electricians at greater risk because of this difference?
Since the first electrical code book, “Rules and Requirements of the Underwriters Association of the Middle Department for the Installation of Wiring and Apparatus for Light, Heat and Power,” dated Aug. 31, 1897, automatic cut-outs (i.e., fuses and circuit breakers) “must be placed on all service wires … as near as possible to the point where they enter the building… .”
In past editions of the National Electrical Code (NEC) — through the 1984 edition — ungrounded service conductors were required to be provided with overcurrent protection. It wasn’t until the 1978 NEC that the term “overcurrent” was formally defined in Art. 100. Also added to the 1978 NEC was the definition of “overload.” The following definitions can be found in the 2011 edition of the NEC.
“Overcurrent: Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault.”
“Overload: Operation of equipment in excess of normal, full load rating, or of a conductor in excess of rated ampacity that, when it persists for a sufficient length of time, would cause damage or dangerous overheating. A fault, such as a short circuit or ground fault, is not an overload.”
Beginning with the 1987 NEC, ungrounded service conductors were required to be provided with a different type of protection; that is, overload protection instead of the previously required overcurrent protection.
So what’s the difference between overload and overcurrent protection? Without much thought, you might assume there is little, if any, distinction between these two terms. The average Code user knows fuses and circuit breakers provide overcurrent protection to most circuits. But of course, a closer reading of these two definitions points out major differences between the terms as they relate to protection from higher-than-allowed electrical currents.
Giving this some thought, locating properly sized overcurrent protection devices at the beginning of a circuit provides good protection downstream from short circuits, line to ground faults, and overloads — no matter if circuit breakers or fuses are used. Thus, any one of these three cases (short circuits, line-to-ground faults, and overloads) is possible — and each one will cause quick and automatic full circuit disconnection using either fuse(s) or circuit breakers as overcurrent protection.
The placement of a properly sized overcurrent protective device at the beginning of a circuit allows a circuit breaker or fuse to operate or open the circuit interrupting all current flow to the faulted conductor(s). This scenario meets the definition of overcurrent and demonstrates required overcurrent protection required for a feeder or a branch circuit. Re-reading these NEC definitions, note this type of protection clearly is overcurrent protection — the type of protection required for equipment covered by the general rules of Art. 240.
But moving on to Art. 230, you see in 230.90 that each ungrounded service conductor must have overload protection (not overcurrent protection) and must be protected by a fuse or circuit breaker rated not higher than the allowable ampacity of the conductor. This required overload protection is placed most often within the service disconnecting means. Remember, the service disconnecting means is generally placed at the end of the service conductors, and there’s no requirement for overcurrent protection ahead of the service conductors.
Inspecting a typical service circuit, notice high currents caused by a short circuit or a line to ground fault in the service conductors can’t be sensed by ordinary downstream overcurrent devices located in the service disconnect. In fact, the only overcurrent that can be sensed directly on the service conductors is an overload current. Thus, properly sized overcurrent devices can provide overload protection but not short circuit or line-to-ground fault protection for service conductors.
Let’s try to visualize the previous statements by working through a potential job activity on an older house.
Suppose you use both types of circuits; one circuit is protected according to Art. 240, and a second is protected according to Art. 230. The location is a city dwelling. One circuit is a service cable connected to the electric utility service drop, which supplies the electric utility meter and then runs inside the house and connects to the service disconnect. The other circuit is a service type cable used as a feeder that supplies the garage. Both circuits are located outdoors, run in service-entrance cable, and mounted on the exposed wood surface of the structure (Photo).
Now suppose these circuits need to be moved due to a siding repair job. If an untrained worker attempted to remove old rusted 2-hole support clips that fastened each cable to the wooden surface of the structure — and the claw of his wooden hammer and the cable support clip caused a short circuit in the cable that was about 12 in. from the worker’s face — what would happen?
Typically, the short circuited service cable supplying the house would likely create a large ball of flame. This short-circuited service cable may also be slow to de-energize and could continue to burn for a period of time. However, the short circuited service cable supplying the garage would likely de-energize quickly and hopefully cause only a medium-sized spark that quickly subsided.
The lesson learned from this hypothetical scenario is that identical overcurrent devices provide different types of overcurrent protection, simply by where they are physically placed within the circuit — and that placement substantially alters the risk of injury to a worker. Training electrical workers to see the real safety differences between overcurrent protection and overload protection should provide a deeper comprehension of safe work practices and procedures.
Sheehan is a curriculum specialist with the NJATC based in Upper Marlboro, Md. He can be reached at firstname.lastname@example.org.