Overheating is one of the single biggest causes of failure in critical low-voltage switchgear, such as Moulded Case Circuit Breakers (MCCBs) and Air Circuit Breakers (ACBs). Modern circuit protection is provided by digital, microprocessor-based overcurrent relays which are operated by measuring current flow, not heat. Therefore, abnormal heating caused by loose connection bolts, ventilation failure or worn contacts is usually left undetected by conventional circuit protection devices. This abnormal heating can lead to premature failure of switchgear and also constitutes a potential fire hazard.
Many power-critical applications such as data centres, hospitals, utilities and mining sites use technology such as thermal imaging to help identify overheating in switchboards, but this method is only valid for a specific ‘snapshot’ in time.
Recent developments within microprocessor-based circuit breakers provide the option of monitoring the temperature of the contacts and terminals continuously, which can substantially minimise downtime and reduce the risk of fire.
If an abnormal condition was detected, this information would be communicated to the Building Management System (BMS) to enable the facility managers to plan for preventative maintenance before a critical power outage or fire incident occurred.
NHP is proud to bring a system unique to Terasaki ACBs that provides condition-based temperature monitoring as an option.
This fully integrated temperature condition monitoring system continually checks for overheating abnormalities that could be due to an issue with the main conductors, contacts and connections – or as this temperature monitoring concept is known within the European electrical industry, 3C.
The status and wear of the contacts is determined by the temperature measurement using NTC (Negative Temperature Coefficient) thermistors. Self-diagnosis is achieved by direct measurement of physical properties. Each phase of the circuit breaker contact is fitted with its own thermistor.
The thermistor will analyse all three phases continuously, every ten milliseconds. The thermistors used are in a glass encapsulated package, diode outline, with axial tinned dumet (Copper-Clad Ni-Fe) wire.
Continuous temperature monitoring
It is important to distinguish between direct, continuous measurement and algorithmic modelling contact wear indications. The latter is inherently less accurate and therefore more likely to result in false alarms or reduced protection.
Direct continuous monitoring of the contact temperature provides valuable input for preventative and predictive maintenance programs.
Should an abnormal temperature occur, the ACB’s integrated protection relay will generate an overheat alarm on the panel-mounted LCD screen, close a volt-free output contact and deliver a message to the Modbus network.
To complement the integrated circuit breaker over temperature protection and extend the level of monitoring to other areas of the switchboard, such as busbar zones, external 3C modules can be used.
These external modules use non-conductive fibre-optic probes which can be fixed directly to a busbar via a special terminating lug. The temperature is measured by pulsing a laser down the fibre-optic cable to a phosphor dot located on the terminating lug.
The phosphor dot is excited by the laser and starts to glow. The module then measures how long the phosphor glows (which is linked to the temperature) and then calculates the temperature that can be communicated over the network.
This information enables facility managers to plan for any necessary maintenance. The implementation of condition monitoring techniques such as this can be equally applied to older installations as well as new builds.
End customers considering retrofit solutions to replace aging switchgear can also take advantage of new solutions such as this when considering the replacement or upgrade of their protection and switchgear.
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