Chapter 2
Compliance with OSHA involves adherence to the following:
1. A facility must provide, and be able to demonstrate, a safety program with defined responsibilities.
2. Calculations for the degree of arc flash hazard.
3. Correct personal protective equipment (PPE) for workers.
4. Training for workers on the hazards of arc flash.
5. Appropriate tools for safe working.
2.2.8.1 Warning Labels on Equipment
The National Electric C de e e a e abe c a e e e f a protection boundary, its incident energy level and the required personal protective equipment (PPE).
High Voltage Hazards and Protective Equipment
2.2.8.2 Arc Flash Prevention
It is important to carry out an arc flash hazard analysis to identify the presence and location of potential hazards. To perform the arc flash hazard analysis, the following details of the electrical installation are required:
1. The short-circuit currents are calculated;
2. The risk area and the energy released by the arc (the formulas are given by NFPA and IEEE) are calculated; these values depend on the trip time of the protection functions and on the short-circuit values;
3. The risk category is defined to determine the minimum requirements for the personal protective equipment (PPE).
2.2.8.3 Arc Flash Analysis Input
1. Short circuit current value for a bolted fault Ik 2. A protective equipment scheme.
2.2.8.4 Arc Flash Analysis Output
1. The flash protection boundary Dc, the distance from live parts within which a person could receive a second-degree burn, if an electrical arc was to occur;
2. The incident energy E Ik
Short Circuit Current
Arc Flash Hazard Analysis
Dc
Risk Category and Protective boundary
2.2.9 Fault Current Calculation
The prospective fault current or short circuit current is the highest electric current which can exist in an electrical system under short-circuit conditions. It is determined by the voltage and impedance of the supply system. The magnitude of the fault current is determined by the total impedance of the generator, the cable, the transformer and the fault. This heavy current
Chapter 2
G
M
0.025
MSB
0.010
0.015 DB
Fault Location
Figure 2.12 – Fault Current Calculation
A 6500 V 2000 W 0.8 C , e ad c e = 2000,000 / (1.73 6500 0.8) = 222 A The total impedance at the fault ca = 0.025 + 0.010 + 0.015 = 0.05
The short circuit fault current = 6500 / 0.05 = 130 kA
Similarly, the short circuit fault current at the DB = 6500 / (0.025 + 0.010) = 186 KA Also, at the MSB the short circuit fault current = 6500 / 0.025 = 260 KA
2.2.10 Incident Energy
It is the energy per unit area that is received on a surface located at a working distance away from the arc flash location. Incident energy is measured in calories per cm2 or joules per cm2. Incident energy is both radiant and convective. It is inversely proportional to the square of the working distance. It is directly proportional to the duration of the arc and to the available fault current.
Figure 2.13 – Incident Energy Effects
Very high
Incident Energy
Much lower incident energy due to increased distance from arc source
High Voltage Hazards and Protective Equipment
2.2.10.1 Incident Energy Calculation
E = 1038.7 D-1.4738 x T[0.0093F2 0.3453F + 5.9675]
E = Incident Energy in cal / cm2 D is the distance to the arcing point T is the time to clear the arcing fault F is the available short circuit current 2.2.10.2 Incident Energy and Damage Level
Incident Energy (cal / cm2) Degree of burn
1.2 2nd degree burn to bare skin
4 Ignite a cotton shirt
8 3rd degree burn to bare skin
Table 2.3 – Incident Energy and Damage levels 2.2.10.3 Hazard Risk Category
It is the level of arc flash protection clothing a person must wear to protect oneself against a minimum level of incident energy and is measured in calories per centimeter squared. The NFPA has identified four FR hazardous risk category levels, which are numbered by severity from 1 to 4 as mentioned in Table 2.4.
Incident Energy Level (cal per cm2)
Hazard Risk Category
0 to 1.2 0
1.21 to 4 1
4.1 to 8 2
8.1 to 25 3
25.1 to 40 4
Table 2.4 – Hazard Risk Categories
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2.2.11 Approach / Protection Boundaries
Any point on an exposed, energised electrical conductor or circular part
No parts of the body may enter the prohibited space.
Keep as much of the body out of the restricted space as possible
Conditions to be cross the restricted approach
boundary:
1) Qualified person
2) Documented approved authorized plan.
3) Appropriate personal protective equipment (PPE)
To cross the limited approach
boundary, you must be a qualified
person
To cross the flash boundary,
approach flash flame protection equipment must be used.
Electrical box or system
Prohibited approach boundary
Restricted approach boundary
Limited approach boundary
Flash protection boundary from
calculations
Figure 2.14 – Approach Boundary related to Exposed HV live Conductor
The National Fire Protection Association (NFPA) has developed specific approach boundaries designed to protect employees while working on or near energized equipment.
These boundaries are:
2.2.11.1 Flash Protection Boundary (Outer Boundary)
It is the farthest established boundary from the energy source. If an arc flash occurred, this boundary is where an employee would be exposed to a curable second-degree burn at about 1.2 calories / cm2. The issue here is that the heat generated from a flash results in burns.
2.2.11.2 Limited Approach
It is a distance from an exposed live part where a shock hazard exists.
High Voltage Hazards and Protective Equipment
2.2.11.3 Restricted Approach
It is a distance from an exposed live part where there is an increased risk of shock.
2.2.11.4 Prohibited Approach (Inner Boundary)
It is the distance from an exposed part considered the same as contacting the live part.
2.2.12 Expected Proximity of Hands and Tools to Live LV Conductors
Flash protection boundary can be
greater than or less than the limited approach
boundary
Limited approach boundary
Restricted space
Enclosure
Exposed Energised Conductors of
Circuit Part
Prohibited Zero Distance 7 Inches
26 Inches 60 Inches
Restricted approach boundary Limited space
Figure 2.15 – Expected Proximity to Live Conductors Based on Competence and Safe Working Distances 2.2.13 PPE for Arc Flash Hazards
It is very important to wear the appropriate arc protected personal protective equipment to prevent a possible arc flash.
2.2.13.1 Requirements of PPE Layering
The outer layers must be flame resistant The under layers must be non-melting
Fit The clothing must fit properly (loose), with least interference
Chapter 2
Hazard Risk Category 0 0 to 1.2 cal per cm2
a) 100% cotton long sleeve shirt and long pants
b) Safety glasses c) Hearing protection
d) Leather and insulated gloves (as required)
e) Leather work boots
Hazard Risk Category 1 1.21 to 4 cal per cm2
a) 4+ cal long sleeve shirt &
long pants (or) coveralls b) Hard hat
c) Safety glasses
d) Arc rated face shield e) Hearing protection
(inserts)
f) Vulcanised rubber gloves g) Leather gloves
h) Leather work boots
Hazard Risk Category 2 4.1 to 8 cal per cm2
a) 8+ cal long-sleeved shirt and long pants (or) coveralls
b) Hardhat
c) Safety glasses
d) Arc-rated face shield e) Hearing protection
(inserts)
f) Vulcanised rubber gloves
g) Leather gloves h) Leather work boots
Hazard Risk Category 3 8.1 to 25 cal per cm2 a) 25+ cal flash suit with
a hood over a 100%
cotton long-sleeved shirt and long pants b) Safety glasses
c) Arc-rated face shield d) Hearing protection
(inserts)
e) Vulcanised rubber gloves
f) Leather gloves
g) HV Insulated Rubber work boots
High Voltage Hazards and Protective Equipment
Hazard Risk Category 4 25.1 to 40 cal per cm2
a) 40+ cal flash suit with hood over flame resistant long- sleeved shirt and long pants b) Safety glasses
c) Arc-rated face shield
d) Hearing protection (inserts) e) Vulcanised rubber gloves f) Leather gloves
g) HV Insulated Rubber work boots
Opening the front door will change the hazard risk category
2.2.13.2 Voltage-Rated Gloves
Electrical gloves are the first line of defence against electrical shock. Leather protectors must be worn over the rubber gloves which will protect them against the thermal effect of arcs as the rubber is vulnerable to cracking due to the effects of ozone.
Note: Leather protectors must be worn over the rubber gloves
Figure 2.16(a) – Voltage-rated Gloves
40 Cal/cm2
Chapter 2
Electrical Gloves are classified as:
Class 00 - up to 500 V AC Class 0 - up to 1000 V AC Class 1 - up to 7500 V AC Class 2 - up to 17000 V
Figure 2.16(b) – Types of Electrical Gloves
Class Tag Color Proof Test Voltage AC /
DC
Maximum Use Voltage AC / DC
00 Beige 2,500 /10,000 500 / 750
0 Red 5,000 / 20,000 1,000 / 1,500
1 White 10,000 / 40,000 7,500 / 11,250
2 Yellow 20,000 / 50,000 17,000 / 25,500
Table 2.5 – Classification of Electrical Gloves
Gloves must be tested before the first issue and consequently every 6 months. If they are tested, but not issued for service, the gloves may not be put into service unless they are tested within the previous year.
High Voltage Hazards and Protective Equipment
Figure 2.17(a) – Roll-up Test
Figure 2.17(b) – Inflator Test Before Use 2.2.13.3 Electrical Insulating Matting (IEC 61111-2009)
Electrical safety mats are important safety equipment. Safety mats protect personnel from electric shock by providing an insulated surface to work on.
Figure 2.18 – Electrical Insulating Matting Class 0
With a working voltage of 1000 V AC, it is 6 mm thick. It has a fine ribbed pattern.
Class 1
With a working voltage of 7500 V AC, it is also 6 mm thick and has a fine ribbed special rubber, non-slip pattern.
Class 2
Chapter 2
2.2.14 Arc Flash Detector
In modern protection systems, the need to operate in a few milliseconds is typically met by detecting the light from an arc flash and initiating tripping action via solid-state tripping elements. This approach is recognized in the IEC standard 62271-200. Response time is the key as an arc develops and becomes destructive within milliseconds. Failure to open a circuit breaker in time can result in enormous losses.
Figure 2.19 – An Arc Detecting System
The damage resulting from an arcing accident relates directly to the amount of current flowing through the short and the time duration. However, of the two parameters, time duration is the more critical. Thus, to maximize protection, both the arc flash detector and the entire switchgear system must have a quick response time.
An arc flash detection relay takes advantage of this phenomenon to achieve significantly faster response times - thereby affording significantly greater protection from damage than the conventional relay. Thus, arc flash detection has become a critical requirement for all switchgear installations.
Arc Detection Relay
High Voltage Hazards and Protective Equipment
However, light is only one of many indications that an arc flash has occurred. A microprocessor-based high-speed relay sends trip signal to the breaker upon sensing a light flash with high speed light sensors installed in the switchgear compartments. The intense light associated with an arc is detected by the arc detector. A current sensing unit can be provided to verify that there is also an arcing current.
Figure 2.20 – An Arc Detection Unit
The arc monitor will react in around 2 ms, sending a trip signal to the circuit-breaker to clear the arc. The breaker clearing time is critical to provide personnel protection which can be as high as 100 ms. Fibre optics, with its inherent speed and EMI immunity, make it a perfect medium for an arc flash detection system.
Chapter 2
Emitter Detector
Loop Sensor
Bare Fibre Jacketed
Fibres
Figure 2.21 – A loop Sensor
The optical detector unit includes an optical emitter and receiver, an optical sensor in the form of a bare fibre loop and fibre optic cable. The optical sensor collects the flash light and transfers it via a fibre optic cable to the fibre optic receiver, which converts the optical signal to an electrical signal that informs the control system when an arc flash is occurring.
There are two types of optical sensors commonly used in such systems namely the point sensor and the loop sensor. The point sensor approach uses a light sensor and an optical receiver to detect light in each area, while the loop sensor uses a loop of bare fibre positioned strategically throughout the equipment as shown in Figure 2.21 above.
2.2.14.1 Wavelength and Illumination
Generally, the wavelength range of an arc flash is 300-1500 nm compound light.
Therefore, a 650-nm or 820-nm fibre optic receiver can be used to detect the arc flash light.
Two types of fibre optic cables can be used within this wavelength range namely plastic optical fibre (POF) or a multimode glass fibre cable.
A POF cable is best suited for 650 nm since it has the lowest attenuation at this wavelength. A POF cable is also cheaper and is easier to install than other types of fibre optic cables. A multimode glass fibre cable has lower attenuation at 820 nm the POF cable has at 650 nm.
2.2.15 The Best Way to Prevent the Hazards of High Voltage Stop - Before Action
Think - Risks / Hazards
Options - Prove the circuit dead / alive Protection Wear proper PPE