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Home / Technical Articles / Maintenance and testing of overcurrent protective devices

Electrical system reliability

When designing electrical distribution systems, required maintenance and testing of the overcurrent protective devices is a very important consideration.

Maintenance and testing of the overcurrent protective devices
Maintenance and testing of the overcurrent protective devices

The electrical system reliability, component and circuit protection, and overall safety are directly related to the reliability and performance of the overcurrent protective device and can depend upon whether the required testing and maintenance are performed as prescribed for the overcurrent protective device utilized.

The required maintenance and testing of the system can depend upon the type of overcurrent protective device selected.

Circuit Breakers

Many engineers and owners view molded case circuit breaker systems as “easy”…just install it, reset the devices if needed and walk away. However, periodic testing and maintenance of circuit breakers is extremely important to the system reliability and protection.

Schneider Electric's 3-phase molded case circuit breaker type NSX 630A
Schneider Electric’s 3-phase molded case circuit breaker type NSX 630A


NFPA 70B – Recommended Practice for Electrical Equipment Maintenance indicates that testing and maintenance of molded case circuit breakers should be completed every 6 months to 3 years, depending upon the conditions of use.

This includes typical maintenance such as:

  • Tightening of connections,
  • Checking for signs of overheating, and
  • Checking for any structural defects or cracks.
Manual operation of the circuit breaker is typically recommended to be completed once per year. Testing of molded case circuit breakers to assure proper overcurrent protection and operation is also recommended during this period.

This includes removing the circuit breaker and verifying the protection and operation for overloads (typically 300%) with the manufacturer’s overcurrent trip data. Additional molded case circuit breaker (MCCB) testing of insulation resistance, individual pole resistance, rated hold-in, and instantaneous operation are recommended by NEMA and may require special testing equipment.

It is important to realize that if a deficiency is discovered during testing and maintenance, the only solution is to replace a molded case circuit breaker because adjustments or repairs cannot be made to this type of device.

In addition, replacement is typically recommended after the molded case circuit breaker has interrupted a short-circuit current near its marked interrupted rating. This process results in additional expenses and may involve delays in finding a replacement device.

Per NFPA 70B, testing and maintenance of low-voltage power circuit breakers is even more expansive and can be required after tripping on an overcurrent condition. It is important to realize that the maintenance and testing of these devices can only be completed by a qualified person.

Often special testing companies are used for this purpose or the device must be sent back to the manufacturer, requiring spare devices during this period.

The question is, how often is this completed?

In commercial installations, the answer is probably never. This lack of maintenance and testing can adversely affect the reliability and protection capabilities during overcurrent conditions in the electrical distribution system.


NFPA 70B recommends checking fuse continuity during scheduled maintenance, but testing to assure proper operation and protection against overcurrent conditions is not required.

Fusible switches and fuse blocks require maintenance, such as tightening of connections and checking for signs of overheating as recommended per NFPA 70B.

A fused 3-phase safety switch serves as the PV service disconnect at a site employing a supply side connection
A fused 3-phase safety switch serves as the PV service disconnect at a site employing a supply side connection

Resetting Overcurrent Protective Devices

As mentioned previously, circuit breakers are sometimes selected over fuses because circuit breakers can be reset where fuses have to be replaced. The most time consuming activity that results from the operation of the overcurrent protective device is typically investigating the cause of the overcurrent condition.

A known overload condition is the only situation that permits the immediate resetting or replacement of overcurrent protective devices per OSHA. If the cause for the operation of an overcurrent protective device is not known, the cause must be investigated.

Thus, having a device that can be easily reset, such as a circuit breaker, possibly into a fault condition, could be a safety hazard and a violation of OSHA regulations.

Because a fuse requires replacement by a qualified person, it is less likely to violate OSHA. Also, when an opened fuse is replaced with a new fuse in the circuit, the circuit is protected by a new factory calibrated device.

Generally, overload conditions occur on branch-circuit devices. Typically this is on lighting and appliance circuits feed from circuit breaker panelboards, where resetting of circuit breakers may be possible. Motor circuits also are subject to overload considerations.

However, typically the device that operates is the overload relay, which can be easily reset after an overload situation. The motor branch-circuit device (fuse or circuit breaker) operates, as indicated in NEC® 430.52, for protection of short-circuits and ground-fault conditions. Thus, if this device opens, it should not be reset or replaced without investigating the circuit since it most likely was a short-circuit condition.

Overcurrent conditions in feeders and mains are typically the result of short-circuits and are infrequent. Because they are most likely short-circuits, the circuit should be investigated first before resetting or replacing as well.

Also, if a feeder or main is protected by a circuit breaker that has opened, the circuit breaker should be examined and tested to be sure it is suitable to be placed back in service.

Reference // Electrical Plan Review – Overcurrent Protection and Devices, Short-Circuit Calculations, Component Protection, Selective Coordination, and Other Considerations – COPPER BUSSMANN

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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Yana
    Apr 22, 2019

    Does anyone know why maintenance testing is so popular?

  2. bussmannfusesupply
    Aug 15, 2015

    Bussmann/Eaton offers free samples of their new class J fuse block to ensure their clients of its power distribution capability.

  3. Imran Albalushi
    Jun 11, 2015

    Nice very Informative……

  4. ramesh
    Jun 09, 2015

    greetings, great work sir

  5. Ghosoun Hashem
    May 09, 2015

    I like your web, and want to thank u for your great efforts.

  6. Jack Rose
    May 09, 2015

    I recall reading of a device a few years back that would supply the current to trip an OCPD and then verify that the device did function properly. Are you aware of a test instrument that can accomplish this?

    Thanks, Jack

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