Читать книгу Arc Flash Hazard Analysis and Mitigation - J. C. Das - Страница 44

The protection system, coordination and suitable selection of additional protective devices can reduce the arcing times. Examples are differential protection, light sensing supervised by current signature, zone interlocking, arc flash maintenance switches which bring alternative relay settings into operation.

In some cases the arc fault current can be low resulting in increased fault clearing time. For example, the fault on the secondary side of a transformer, which is used by the transformer primary protection device. Due to ratio of transformation this current will be reduced resulting in prolonged clearing time. A secondary protective relay can be applied which will transfer trip primary proactive device.

Well-planned procedures, work practices and training can avoid arc flash hazards. Remote racking, remote control panels for switching, supervisory controls can help worker for safe operation from a distance outside the arc flash boundary. Infra-red windows, partial discharge sensors can be very helpful diagnostic tools.

Arc-resistant equipment can be used.

TABLE 1.14. Measures to Protect Against Arc Flash

Mitigation	Type	Advantages	Disadvantages	Recommendations
Compartmentalization (IEC type-4b or ANSI equivalent barriers and boots	Prevention	Reduces the likelihood of arc flash by compartmentalization and insulating energized parts	Equivalent to a guard on a saw. Hazard is still present. Maintenance and testing may require exposure to energized parts	It does not reduce the energy available, it does reduce the probability of a fault. Compartmentalization or change incident energy calculation.
Arc-resistant equipment	Containment (passive)	Protects the worker by directing energy away from the worker when required covers are on. Tested per IEEE Std. C37.20.7	Maintenance and testing may require removal of covers. Equipment is exposed to damage and has a larger footprint. May require second migration means to stay within rated arcing duration especially at LV	This measure offers protection for specific tasks, but does not reduce available energy. This measure is recommended where active mitigation methods are not acceptable.
High-speed shorting switch	Active (reduction)	Fights fire with fire. Re-directs arc flash energy into the bus by creating an intentional bolted fault; may use optical, differential and current waveform recognition.	Unconventional method and requires additional equipment.	May qualify as arc resistant per IEEE Std. 37.20.7. Needs robust controls to minimize chances of nuisance operation.
ZSI (blocking schemes)	Active (reduction)	Communication-aided transfer trip scheme that reduces breaker trip time and amount of energy released	Requires communication wiring between breakers. Trip units or relays must be installed with ZSI capability	Recommended for LV applications. Also available with some MV relays.

TABLE 1.15. Matrix of Sample Arc Flash Mitigation Measures

Technology	Arc-Flash Mitigation	Incremental Commissioning Complexity	Protection for Intentional Exposure	Protection for Incidental Exposure	Foot-print	Capital Expense	Operational Expenditure	Equipment Survivability	Arc Flash Incident Energy Reduction	Typical Application
Prevention	Barriers	Low	Limited	Limited	No change	Low	Low	Moderate impact	None	LV switchgear switchboard
	ANSI compartmentalization	Low	Limited	Limited	No change	Low	Low	Moderate impact	None	LV switchgear switchboard
	IEC-4b compartmentalization	Low	Limited	Limited	No change	Low	Low	Moderate impact	None	LV switchgear switchboard
	HRG	High	Limited	Limited	Increased	Medium	Low	Moderate impact	SLG fault only	480V to 5 kV
	Shielding barriers	Low	Limited	Limited	No change	Low	Low	No impact	None	LV arc-resistant switchgear
	Adm. controls	N/A	Limited	Limited	No change	Low	Low	No impact	None	All, training requirement
Containment	Arc-resistant equipment	Low	No	Yes	Increased	High	Low	No impact	None	LV/MV switchgear
	Solid dielectric bus	Medium	Yes	Yes	No change	Low	Low	Moderate	None	MV switchgear
Reduction	Differential protraction	High	Yes	Yes	No change	High	Medium	impact	Significant	LV/MV switchgear
	ZSI	Medium	Yes	Yes	No change	Low	Medium	Moderate	Significant	LV switchgear/ switchboard
	Transfer trip	High	Yes	Yes	Increased	High	Medium	impact	Significant	LV switchgear fed from transformers of >750 kVA

TABLE 1.16. Measures for Mitigating Arc Flashes at Key Locations in Data Centers

System Location	Issue	Recommendations
LV service entrance equipment	Protection on HV side of transformer does not respond quickly for LV faults	Transfer trip, differential protection along with ZSI
UPS input/output switchboards	Selectivity requirements lead to extended clearing times	ZSI or optical detection
PDU secondary (480 V to 208/120 V)	Low fault levels and P combined with transformer inrush currents extend arc-clearing times	Maintenance switch, compartmentalization of PDU with administrative controls
Generator paralleling equipment	Low fault levels combined with multiple sources extend arc-clearing times	Bus differential or optical detection, adaptive relay settings
MV distribution equipment	Multiple utility sources and/or generator sources result in high fault currents	High-speed shorting switch, bus differential, and optical relaying

Figure 1.11. An outfit for PPE HRC4.

Figure 1.11 shows a PPE outfit for HRC=4. It looks like a space suit and hampers the mobility of a worker to render fine tasks. The arc flash energy must be reduced to no more than 8 cal/cm² with appropriate measures.