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Maintenance of low voltage switchgear

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IEC60364-6 requires initial and periodic verifications of electrical installations. The electrical switchboard and all its equipment continue to age whether they operate or not. This aging process is due mainly to environmental influences and operating conditions. To ensure that Low voltage switchgear and especially circuit breaker retains the operating and safety characteristics specified in the catalogue for the whole of its service life, it is recommended that:

  • The device is installed in optimum environmental and operating conditions
  • Routine inspections and regular maintenance are carried out by qualified personnel.

Parameters influencing the ageing

A device placed in given conditions is subjected to its effects. The main factors that accelerate device ageing are:

  • temperature
  • vibration
  • relative humidity
  • salt environment
  • dust
  • corrosive atmospheres.
  • percent load
  • current harmonics

Preventive maintenance

Preventive maintenance consists in carrying out, at predetermined intervals or according to prescribed criteria, checks intended to reduce the probability of a failure or deterioration in the operation of a system.

There are two types of preventive maintenance:

Periodic maintenance

For each type of product, maintenance recommendations have to be formalized in a dedicated document by the technical department. These verification procedures, intended to maintain systems or their subassemblies in correct operating condition over the targeted service life, must then be carried out according to the time intervals stipulated in this document.

Conditional maintenance

To a certain extent, conditional-maintenance operations are a means to reduce (but not eliminate) the recommended periodic-maintenance operations (thus limited to the strict minimum) that require an annual shutdown of the installation.

These operations are launched when programmed alarms indicate that a predefined threshold has been reached. (Number of operation > durability, aging indicators…)

Electronic trip units in power circuit breaker can propose such functions. Conditional maintenance is the means to optimise installation maintenance.

Example of maintenance recommendation for Power Circuit Breaker (>630A)

The table below indicates maintenance operations and their intervals:

Interval Maintenance operations
1 year Visual inspection and functional testing, replacement of faulty accessories
2 years + servicing operation and subassembly tests
5 years + diagnostics and repairs (by manufacturer)

Fig. H63Recommended periodic maintenance operations, for normal operating conditions

The intervals stated are for normal environmental and operating conditions.

Provided all the environmental conditions are more favourable, maintenance intervals can be longer.

If just one of the conditions is more severe, maintenance must be carried out more frequently.

Functions linked specifically to safety require particular intervals. For example, emergency switching and earth leakage protection

The case

The case is an essential element in the circuit breaker.

First of all, it ensures a number of safety functions:

  • functional insulation between the phases themselves and between the phases and the exposed conductive parts in order to resist transient overvoltages caused by the distribution system
  • a barrier avoiding direct user contact with live parts
  • protection against the effects of electrical arcs and overpressures caused by short-circuits.

Secondly, it serves to support the entire pole operating mechanism as well as the mechanical and electrical accessories of the circuit breaker.

On the case, there should be:

  • no traces of grime (grease), excessive dust or condensation which all reduce insulation
  • no signs of burns or cracks which would reduce the mechanical solidity of the case and thus its capacity to withstand short-circuits.

Preventive maintenance for cases consists of a visual inspection of its condition and cleaning with a dry cloth or a vacuum cleaner. All cleaning products with solvents are strictly forbidden. It is advised to measure the insulation every five years and following trips due to a short-circuit. The product must be replaced if there are signs of burns or cracks.

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Arc chutes (for Air Circuit breaker)

During a short-circuit, the arc chute serves to extinguish the arc and to absorb the high level of energy along the entire path of the short-circuit. It also contributes to arc extinction under rated current conditions. An arc chute that is not in good condition may not be capable of fully clearing the short-circuit and ultimately result in the destruction of the circuit breaker. The arc chutes for air circuit breaker must be regularly checked. The fins of the arc chutes may be blackened but must not be significantly damaged. What is more, the filters must not be blocked to avoid internal overpressures. It is advised to use a vacuum cleaner rather than a cloth to remove dust from the outside of the arc chutes.

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Main contacts (for Air Circuit breaker)

The contacts make and break the current under normal conditions (rated current for the installation)and under exceptional conditions (overloads and short-circuits). The contacts are eroded by the many opening and closing cycles and can be particularly deteriorated by short-circuit currents.

Worn contacts may result in abnormal temperature rise and accelerate device ageing.

It is imperative to remove the arc chutes and visually check contact wear at least once a year and following each short-circuit.

The contact-wear indicators constitute an absolute minimum value that must not be overrun.

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Device and chassis mechanisms

Mechanical operation of the circuit breaker may be hindered by dust, knocks, aggressive atmospheres, no greasing or excessive greasing. Operating safety is ensured by dusting and general cleaning, proper greasing and regular opening and closing of the circuit breaker.

The imperative need to ensure continuity of service in an installation generally means that power circuit breakers are rarely operated. If, on the one hand, an excessive number of operating cycles accelerates device ageing, it is also true that a lack of operation over a long period can result in mechanical malfunctions. Regular operation is required to maintain the normal performance level of each part involved in the opening and closing cycles.

In installations where power circuit breakers are used in source changeover systems, it is advised to periodically operate the circuit breaker for the alternate source.

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Electronic trip unit

If an electric fault occurs in the installation, the electronic trip unit detects the fault and orders the circuit breaker to open and thus protect life and property. Electronic components and circuit boards are sensitive to the environment (ambient temperature, humid and corrosive atmospheres) and to severe operating conditions (magnetic fields, vibrations, etc.). To ensure correct operation, it is necessary to periodically check:

  • the chain of action resulting in a trip
  • the response time as a function of the level of the fault current.

Depending on the operating and environment conditions, it is advised to estimate the service life of trip units, and to replace them if necessary to avoid any risk of non-operation when they are needed.

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Auxiliary circuits

Control auxiliaries

MX and XF shunt releases are respectively used to remotely open and close the circuit breaker using an electrical order or by a supervisor via a communication network.

The MN undervoltage release is used to break the power circuit if the distribution system voltage drops or fails in order to protect life (emergency off) or property.

Preventive maintenance consists in periodically checking operation at minimum values. Depending on the operating and environment conditions, it is advised to estimate their service life and to replace them if necessary to avoid any risk of non-operation when they are needed.

Auxiliary wiring

Auxiliary wiring is used to transmit orders to the various control devices and to transmit statuscondition information. Incorrect connections or damaged insulation may result in either nonoperation of the circuit breaker or nuisance tripping.

Auxiliary wiring must be regularly checked and replaced as needed, particularly if there are vibrations, high ambient temperatures or corrosive atmospheres.

Indication contacts

The contacts indicating the status of the circuit-breaker (ON / OFF), of the chassis (CE, CD, CT), a trip due to an electrical fault (SDE) or that the circuit breaker is ready to close (PF) provide the operator with the status information required to react correspondingly. Any incorrect indications may result in erroneous device operation that could endanger life and property. Contact failure (wear, loose connections) may result from vibrations, corrosion or abnormal temperature rise and preventive maintenance must ensure that contacts correctly conduct or isolate according to their positions.

Gear motor

The gear motor (MCH) automatically recharges the operating-mechanism springs as soon as the circuit breaker is closed. The gear motor makes it possible to instantaneously reclose the device following an opening. This function may be indispensable for safety reasons. The charging lever serves simply as a backup means if the auxiliary voltage fails. Given the mechanical forces exerted to charge the mechanism, the gear motor wears quickly. Periodic checks on gear-motor operation and the charging time are required to ensure the device closing function.

Fig. H64Example of maintenance recommendation for Power Circuit Breaker (>630A)