Demystifying arc flash hazard management

Presented by

Senior Electrical Engineer

TERMINOLOGY

Shock Arc

Standard

‘risk of electric shock’ ‘shock hazard analysis’ ‘shock protection boundary’

‘arc blast, arc flash’ AS/NZS 4836

‘arc flash hazard analysis’ ‘flash-protection boundary’ IEEE 1584

‘arc flash hazard analysis’ ‘arc flash boundary’

‘hazard/risk categories’ NFPA 70E

In this presentation:

arc flash = arc blast

System parameters

Arcing current

Working distance

Incident energy

Arc flash hazard category

Flash-protection boundary Arc intensity

Not arc resistant. CB operation with door closed.

Does A-F hazard management apply?

The door was closed

KEY POINT

There is no recognised method of establishing prospective incident energy where an intervening obstruction such as a door is in the path of the arc.

Assess as if door not present?

From NFPA70E-2012 p36

Arcing energy and working distance are identical. The only difference is the nature of the task.

The arc flash hazard categories are different. Why?

KEY POINT

Incident energy does not translate directly to Arc Flash Hazard Category.

b) The desire to avoid providing more protection than is

needed. Hazards may be introduced by the garments such as heat stress, poor visibility, and limited body movement.

From IEEE1584-2002 p4

KEY POINT

Arc Flash Hazard Categories do not correspond to incident energy, they correspond to RISK.

INCIDENT ENERGY OUTCOMES FOR DIFFERENT

SCENARIOS

KEY POINT

Developing an arc flash hazard management strategy is not a purely analytical process. It necessarily involves applying a certain amount of professional judgement and expertise.

ARC FLASH PPE

At the low end, when in doubt default to leather gloves and face shield. There is a large difference between that and nothing. Small impost to cover a small risk. Reinforces good habits.

Racking an LV ACB

KEY POINT

The primary operational control measure for arc flash hazard management is DISTANCE.

Racking an LV ACB

Cross-checker

How far away? ‘First degree burn distance’?

NFPA70E Approach Boundaries Prohibited Approach Boundary Restricted Approach Boundary Arc Flash Boundary Limited Approach Boundary Arc Flash Hazard Shock Hazard From NFPA70E-2012 p24

AS/NZS4836. 3m exclusion zone for LV for other than

DISTANCE

There is no defined answer to the question: How far is safe?

Arc flash boundary is useful to an extent but answers the wrong question.

Suggested approach is to use an exclusion zone of 3m for LV as default, or otherwise roughly double the calculated theoretical arc flash boundary.

A default 3m LV exclusion zone rule is much easier to apply than posting precisely calculated but possibly dubious arc flash

KEY POINT

For bystanders, think in terms of an

exclusion zone

NON-PPE ARC FLASH HAZARD CONTROL

MEASURES

• Prohibit racking by other than qualified electricians • Remote racking & remote switching where possible • Barricading against inadvertent unauthorised approach • Barriers to keep exposed conductors out of arm’s reach • Arc resistant switchgear mandated for new projects

• IP2X within MCC modules mandated for new projects • Safety observer (e.g. AS/NZS4836 Section 6)

KEY POINT

When developing or revising an arc flash hazard management strategy:

ARC FLASH HAZARD MANAGEMENT

Control measures

= f( arc flash hazard category, job-specific hazards )

Arc flash hazard category = f( risk )

Risk = f( consequence, likelihood )

Making significant decisions, e.g. when a safety observer is or isn’t mandated. The methodology matters.

Fuse holder

Task

ACB has been racked out 1. Unbolt lexan Situation Production down One electrician on Issue Is a second electrician required as a safety

ARC FLASH HAZARD MANAGEMENT CONTROL

MEASURES

Safety observer policy. Possible options.

A. Not required when testing for dead. (But what if there has been an isolation error?)

B. Is required. (But could add an extra hours downtime

when it is almost certain that the conductors are actually dead. Defend the policy against challenge.)

TESTING FOR DEAD

Somewhat like Schrödinger’s cat. Neither actually live nor actually dead.

The truth cannot be known without opening the box and doing the test.

Conductors must be treated as if live. But does this mean that the same control measures must be applied as when

they are live? (Noting that opening many types of equipment would not be permitted while actually live.)

Conductors must be treated as live but are most likely dead. The change in likelihood of an incident is several orders of

ARC FLASH HAZARD MANAGEMENT

Safety observer when LV testing for dead. A solution. Apply the principles to establish a methodology

1. Mandate a safety observer for higher level base case categories, e.g. 3 & 4.

2. For LV (not MV/HV) when isolated and testing for dead: Apply the principles behind Informational Note No. 1 and reduce the category by one number to account for the reduction in risk. Y Y N N N SO Required 4 3 2 1 0 Category

Overpressure explosion. Gramercy alumina refinery. 1999

Arc Flash Hazard/Risk categories are a similar concept to IEC61508 Safety Integrity Levels

IEC61508 NFPA 70E Article 130

KEY POINTS

1. There is no recognised method of establishing prospective incident energy where an intervening obstruction such as a door is in the path of the arc.

2. Arc Flash Hazard Categories do not correspond to incident energy, they correspond to RISK.

3. Developing an arc flash hazard management strategy is not purely an analytical process. It necessarily involves applying a certain amount of professional judgement and expertise. 4. The primary operational control measure for arc flash hazard

management is DISTANCE.

5. For bystanders, think in terms of an Exclusion Zone rather than arc flash boundary.