Phase Barriers in MCCBs

Introduction

An electrical breakdown or dielectric breakdown is a process that occurs When electric filed strength accedes than dielectric capacity/ strength of insulating materials or air media. Results, insulating media becomes electrically conductive. The voltage at which a given insulating object becomes conductive is called it's breakdown voltage.

Dielectric strength between conductors depends on following factors:

• Size, shape, and distance between conductors.
• Operating voltage and power quality
• Operating temperature
• Frequency
• Environment conditions: Dielectric strength of air decreases as relative humidity increases
• Climate conditions near installations: Presence of gases and dust, dielectric strength of air ( E.G. Nitrogen, sulfur hexafluoride).

Breakdown occurs quite abruptly (typically in nanoseconds), resulting in the formation of an electrically conductive path and a disruptive discharge between conductors.

Electrical breakdown may be a momentary event (as in an electrostatic discharge), or may lead to a continuous electric arc if protective devices fail to interrupt the current in a power circuit. In this case electrical breakdown can cause catastrophic failure of electrical equipment, and fire hazards. It severely degrades, or even destroys, insulating capability permanently.

For LV switchgear devices like Air Circuit Breakers (ACBs) or Molded Case Circuit Breakers (MCCBs), air acts as an insulating medium between adjacent electrical phases. Dielectric strength of air is 3MV/m. Beyond which air loses its dielectric strength and can result into momentary discharge or continuous electric arcing between conductors or contacts of adjacent phases.

Dielectric breakdown of Air in MCCBs

As we know MCCBs are widely used in residential and industrial applications for the interruption of electrical current upon overcurrent conditions caused by short circuits or by ground faults. In a multi-phase circuit, it is common to have the contact and blade assemblies of the circuit breaker for all phases mechanically linked to each other so that the tripping of any one of any phases will cause the other phases to trip, opening the contacts of all phases. During interruption of electric current, hot ionized electrically conductive gases are produced in the air between the contacts of the individual phases. The ionized gases from one phase may mix with the ionized gases from an adjacent phase outside the housing of MCCB, leading to dielectric breakdown of air and resulting in arcing between the phases. This kind of catastrophic failure can potentially damage the circuit breaker and the equipment that the circuit breakers are designed to protect.

Necessity of Phase Barrier for MCCBs

In addition to the above-mentioned reason, whenever a breaker is tripped, arcing leads to small amounts of carbon and metal particles are deposited on the crossbar / busbar links, increasing the risk of over-surface dielectric breakdown, necessitating the use of a sealant/ inter phase barriers. Phase barrier prevents mixing of ionized gases between phases and maintain dielectric integrity between phases.

Did You know: Inter phase barriers are highly recommended as mandatory accessories for MCCBs. L&T E&A provides interphase barriers as default offering along with the MCCBs.

MCCB Phase Barriers

The phase barrier is positioned circumferentially around the crossbar between each blade carrier and includes a thermoplastic material. The electro-mechanical assembly is housed within a circuit breaker enclosure having an insulating base wall and cover wall. Phase / interpole barriers care about safety and protection of both people and installations, by providing maximum insulation between phases at the terminals of the MCCCB. These interphase barriers reinforce insulation level at the power connection points. They are highly recommended for voltages above 400 VAC for both fixed devices and adapters on plug in base of MCCBs. As system voltage levels go up the severity of arcs increase multifold times, which necessitates installation phase barrier on MCCB. Phase /Interpole barriers for use on both ends of the MCCB are supplied as standard offering by L&T Electrical & Automation. MCCB moulds have been designed to accept phase/ interpole barrier. Interphase barriers are clipped easily between front or rear connection terminals.

Installation of phase barriers

1. Insert a phase barrier inside a slot on MCCB housing present in between two adjacent phases.
2. No Load line biasing for MCCBs.
3. Phase barriers must be mounted on MCCBs before putting them in service.

Installed view of MCCBs with phase barriers

Additional phase barrier is required between 2 MCCBs when they are side by side mounted.

Advantages of using Phase Barrier

1. Insulation between phases - Phase barriers make sure arc does not strike between adjacent electrical phases of MCCB. This helps in avoiding short circuit failures, fire hazards.
2. Increase in creepage distance between phases - Phase barriers are flexible, have low moisture absorption, and have the needed dielectric integrity, leading to reliability. Installation of phase barriers helps in increasing creepage distance and also obstructs ionized gases from adjacent phases from mixing in each other.
3. Low moisture absorption - Phase barrier material exhibits low moisture absorption and hence maintains dielectric integrity throughout the life of MCCB.
4. Installation easiness - Flexible material needs no hardware for installation and uses cavities in the housing of MCCBs for getting mounted.

Conclusion

Phase barriers are perceived as mandatory accessories of MCCBs to avoid short circuit faults and prevent fire hazards. Use of phase barriers are highly recommended for electrical hygiene and safety of devices and personnel.

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