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LV circuit breakers: a yardstick to service life

When determining whether an installed circuit breaker should be replaced or not, safety is the main priority. When fuses were phased out in the 1950s and the changeover to circuit breakers began, the new technology required a deviation from surface-mounted installations towards distribution panels. While there are many installations from this era that are still in daily service, when those properties change hands or repairs have to be carried out, electrical contractors have to issue a Certificate of Compliance that will ensure the continued safe use of the electrical systems in these homes, businesses, factories, and mines.

 

Determination of the predictive life

 

The longevity of a circuit breaker cannot be decided only in terms of the years of use. The service life will require some diagnosis by experts but could be gauged in broad terms by typical manufacturing data that includes environmental factors; tripping and switching cycles; length of service; maintenance levels; as well as inspection and maintenance selection.

 

Correct selection of inspection and maintenance levels can ensure that trouble can be avoided and ensure the circuit breakers can be used for the longest possible time. It is a good idea to introduce periodic inspection approximately a month after commencement of use. Twice yearly inspect that there are no adverse environmental conditions such as steam, corrosion or rust. Every year check for dust and humidity and once every two to three years ensure that the environment is clean and dry. During these inspections examine whether there are any loose connections or damage to components as well as dust or moisture contamination. Look for signs of abnormal temperatures and melted cable insulation or plating; for any discolouring; and for tripping breakers. Ensure that all barriers are in place; and test the earth leakage (EL) operation via the test button or using an EL tester or mechanically trip the circuit breaker.

 

Inspection after tripping or fault

 

The cardinal rule is to establish why the circuit breaker tripped and remove the cause before simply switching on again.  High-level short circuits will have a dramatic effect on the life of a circuit breaker so avoid unnecessary switching into known high-level faults. Before switching on, check the possible reason for tripping, including faulty equipment etc., as well as the estimation of fault level and calculate the length back to the supply transformer.

A low intensity trip will show no signs of carbon soot and the equipment can be re-used after precautionary checks for signs of overheating or tripping. A medium intensity trip will show signs of carbon soot around the venting area only. In this case, the equipment can be reused only once insulation tests >5 MΩ and the equipment has been monitored for signs of overheating or tripping. For a high intensity trip, there will be large amounts of carbon soot in the vent area including metallic residue or soot visually obvious around the handle and damage related to the fault.  In this instance, immediate replacement is required before reconnection.

 

Service life approximations

 

The service life of circuit breakers, therefore, depends on the sum total operational cycles and total tripping cycles on fault conditions and the intensity levels of those faults as well as the impact of the installed environment. This may be gauged by experienced experts who are able to safely open and test the units but this process is neither practical nor cost-effective.

 

Installed conditions, use and environment

Approximate

Service Life

Indoor dry; dust free; clean air; air-conditioned; low risk of tripping; original installed no modifications; regular professional monitoring or maintenance; closed high rated IP panel boards

10-20 years

Indoor light dust; no corrosive gases; low risk of tripping; good service levels of maintenance; thorough inspection if trip out occurred

7-15 years

Dust requiring regular attention; industrial contamination; corrosive gases; moisture; steam; sewage treatment; iron; paper or similar processing and manufacturing plants. Panel boards require constant attention to clean; service and keep production process going. Spontaneous secondary flashovers have occurred in the past;

Highly modified panel boards over five year period.

3-7 years

Excess dust; grit; corrosive moisture  conditions typical of mining; quarry; chemical plants

Typically workers are required to use additional PPE measures to be in the area or are limited in the area. Cable damage is constant in occurrence

1-3 yrs

 

For circuit breakers and units that have been installed in excess of 25 years a systematic replacement is called for. The units have served their intended purpose and non-trip or slow trip may cause additional costly damage. The potential risk of malfunction exceeds the planned replacement costs. The total life cycle has not been under consideration of a single person

 

Summary

 

For continued safe use, circuit breakers of known long installed life, or where the installed conditions are considered arduous, or risk of failure could be considered as not acceptable, require expert evaluation with a view to replacing them. Service staff employees are ideal front line personnel to identify these sites and recommend corrective actions.

 

Testing using Infrared scanners

 

The cost of infrared technology has reached the point where these scanners are being used in many more instances for quick and safe overall evaluation.  The sometimes over-zealous use of this technology needs to take into account a number of important factors.  The first being that circuit breaker design limitations happen at the maximum amp per frame size at full load on specific cable sizes in a free air single unit test. A safe temperature rise of 70°C above ambient can be tolerated on a fully loaded circuit breaker on the terminals by design qualification. In reality, most circuits are never 100% loaded and even less totally balanced between lines. There are natural differential temperatures between load and line and middle and outer terminals. There is also contribution heating from adjacent energized units or surrounding heat sources. For best effect consider maximum load times and maximum ambient. Lastly, the connecting cables provide a heat sink for heat control, do not undersize cables.

 

Energy Efficiency

 

In addition to safety, energy efficiency is an important consideration when choosing a replacement circuit breaker, and saves money in the long run. Over the 25-year average lifespan of a circuit breaker, the lower energy consumption of a reputable circuit breaker will exceed the original purchase price by far.  The total cost of ownership for a circuit breaker consists of more than just the purchase price; the operational costs also need to be taken into consideration.  The operational cost of a circuit breaker is dependent on the efficiency of the circuit breaker.  There are however additional hidden costs: the first being the risk to the installation that may be present if cheap non-compliant circuit breakers are installed.  These devices may fail and ignite under normal load conditions causing damage to an installation and pose a risk to the lives of those who use and occupy the buildings.