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112
PCS100 AVC-20 User Manual
14.1.3
Overload protection
The upstream circuit breaker must provide overload protection to the AVC-20 and downstream loads. The
breaker must be adjusted to trip within the thermal capability of the AVC-20 (or any faster downstream
requirements).
If overloaded the AVC-20 will transition to bypass. The Bypass is rated to carry the following overload:
Maximum overload capacity
(in bypass)
The upstream circuit breaker overload settings must be set to operate within these times to protect the AVC-20,
and instantaneously on any greater current.
14.2 Arc Fault Protection
14.2.1
Arc Fault and Standards
Arc faults may happen due to unexpected occurrences. Arcs can cause lethal injury from burns, pressure and
flying debris. Generally, the risk is in proportion to the energy of the arc which is proportional to the fault current
and time of exposure.
Safety issues in respect of arc faults are considered in the USA National Fire Protection Association NFPA 70E
Standard for Electrical Safety in the Workplace. At this time there is no equivalent European standard. In the
USA, NFPA 70E (and associated standards) mandates:
•
Assessment of whether there are arc flash hazards (if the electrical equipment was de-energized, for example,
the hazard would not be present)
•
Calculation of the energy released by the arc, if present
•
Determination of the flash protection boundary
•
Provision of appropriate personal protective equipment (PPE) for the personnel working within the flash
protection boundary
•
Appropriately label the equipment. These warning labels are placed on the equipment by the plant owner and
not by the manufacturer. The labels shall indicate the minimum protective distance, the energy level which
can be released and the required personal protective equipment (PPE).
Such study is required for all US installations.
NFPA 70E Annex D (referencing references IEEE Standard 1584-2002 - IEEE Guide for Performing Arc Flash Hazard
Calculations) provides guidance on calculation of fault energies, protection boundaries, PPE and labelling.
14.2.2
Arc Fault Protection Recommendation
Arc energy is proportional to current and time so protection must be designed to minimize both. Arc current may
fall between 30 – 70% of the supply fault capacity. Molded case circuit breakers (MCCBs) or air circuit breakers
(ACBs) should be used to promptly isolate energy sources in the event of an arc. Note – to ensure they clear
instantaneously on occurrence of an arc, they must be set to trip immediately on currents of no more than 30% of
the supply fault capacity. Fuses are generally not suitable as fast operation is not assured at the low overload
currents arising in arcs.
14.2.2.1
MCCBs
Where available for the required load current (typically <1600A), molded case circuit breakers (MCCBs) are the
correct choice for protection of PCS100 AVC-20 systems. MCCBs, such as ABB TMax series, use magnetic
repulsion to clear fault currents of 10 -15 times the nominal current within about 6ms. But note that below about
10 times the nominal current, the breakers revert to the electronic trip and are specified as taking < 40 ms to trip.
125% for 10 minutes
150% for 1 minute
500% for 1 s
2000% for 200 ms
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