NFPA70E, arc flash, and safe and efficient thermography practices

Billings, Atlas Inspections; Art Stout, Electrophysics Corp.
Tags: infrared thermography

Estimates indicate that 10 to 15 serious arc flash incidents – those that result in burn injuries requiring treatment in a burn center – occur each day in the U.S., so it is not surprising that awareness of the hazards associated with arc flash continues to grow.

Concerns about operator safety in the event of an arc flash are causing inspectors of high-voltage switchgear to adopt new practices. We will examine the impact of new safety practices – including infrared transparent windows and PPE (personal protective equipment) – on the use of thermography inspection.

What is an Arc Flash?
An Arc flash is essentially a bolt of lightning that occurs around energized electrical equipment. It can occur spontaneously and is often triggered simply by the movement of air when an electrical enclosure is opened.

The National Fire Protection Agency (NFPA) has recognized the significant hazard of arc flash and is attempting to protect workers via the latest implementation of “NFPA 70E - The Standard for Employee Safety in the Workplace".

Every person who has worked around energized electrical equipment is familiar with arc flash – most have seen it first hand. But it is kind of like a major automobile accident: No one really expects it to happen to them, so people have a tendency to drive with significantly less caution than they should.

So it is with arc flash, only worse. Similar to driving, you can make a mistake, or you can be doing everything right when someone slams into you.

Specifically, what is an arc flash? An arc flash is electric current flowing through an arc outside its normal path where air becomes the conductor of high thermal energy (more than 5,000 degrees Celsius) and generates highly conductive plasma.

An arc flash will conduct all available energy and generate an explosive volumetric increase of gases which blows electrical system doors off and potentially generates shrapnel.

What are the causes of arc flash? An arc flash occurs when the gap between conductors or conductors and ground is momentarily bridged. There is always a trigger event which almost always involves human intervention. Typical causes and contributing factors include:

The vast majority of arc faults occur when the door is open or being opened.

The National Fire Protection Agency is the author of NFPA 70, also known as the National Electric Code (NEC). The NEC is an electrical design, installation and inspection standard, and does not specifically address things like electrical maintenance and safe work practices.

A national consensus was needed for safety while working around live electrical equipment. NFPA 70 E is the standard for safe electrical work practices.

NFPA 70 E addresses specific topics: safety-related work practices, safety-related maintenance practices and safety requirements for special equipment. Some of this relates to thermography, which will be addressed later. NFPA 70 suggests that a hazard/risk analysis be conducted prior to working on electrical equipment. The core of the analysis is based on shock and arc flash boundaries which must be done by a qualified electrical engineer.

Shock Hazards, Flash Hazards and PPE Selection
In order to work around live components, the correct personal protective equipment must be determined by carrying out a shock hazard analysis and a flash hazard analysis. Where this is not possible or has not been done, Table 130.7(C)(9)(a) can be used to determine the required PPE based on the task conducted.

In order to work around live components, an Energized Electrical Work Permit must be obtained and should include but not be limited to the following:

NFPA 70 E allows for an exemption to the safe work permit for qualified personnel who are performing tasks such as testing, troubleshooting, voltage measuring, etc., so long as they utilize safe work practices and the correct PPE.

In either case, it is necessary to perform a shock hazard analysis and a flash hazard analysis to help determine safe working practice and proper PPE. A shock hazard analysis will determine the voltage to which personnel are exposed, boundary requirements and the proper PPE necessary to minimize the possibility of shock to personnel. The shock protection boundaries are identified as limited, restricted and prohibited for the distances associated with various voltages.

System Voltage

Limited Approach

Restricted Approach

Prohibited Approach

Up to 750 V

750 V to 15 kV

15 kV to 36 kV

36 kV to 46 kV

3'-6"

5 Feet

6 Feet

8 Feet

1 Foot

2'-2"

2'-7"

2'-9"

1"

7"

10"

17"

Unqualified personnel will be notified and warned of hazards by qualified personnel when working at or near the limited approach boundary. When an unqualified person must work inside the restricted boundary, they will be further notified of the risks and hazards and continuously escorted by a qualified person. Under no circumstances will they be allowed inside the prohibited boundary.

A flash hazard analysis will be conducted in order to protect personnel from being injured by an arc flash. The analysis will determine the flash protection boundary and determine the proper PPE. The flash protection boundary is calculated at the distance from energized parts where a burn will be "recoverable" (second degree) and "incurable" (third degree).

The flash protection boundary for systems that are 600 volts or less shall be 4 feet for clearing times of 6 cycles (.1 second) and available bolted fault current of 50kA or any combination not exceeding 300kA cycles.

For all other clearing times and bolted fault currents, the flash protection boundary will be determined based on the calculated incident energy of an arc fault taking into account system voltage, available current, and clearing time, where incident energy is the measure of thermal energy at a specific distance from the fault.

Selection of PPE by task is allowed in lieu of a flash hazard study. However, for tasks not listed in table 130.7(C)(9)(a) and for clearing times different then those listed there, a complete flash hazard analysis is required.

A selection from table 130.7(C)(9)(a):

600-volt class switchgear (with power circuit breakers or fused switches)

Note: Above 600 V, removal of bolted covers (to expose bare, energized parts) carries a Class 4 (scale 0 to 4, where 4 is determined the riskiest)

Using flash hazard analysis or task risk assessment the following table can be used to identify the correct PPE.

Class

Cal/Cm2

PPE

1

1.0 - 4.0

Cotton underwear, FR pants and LS shirt, hard hat, safety glasses

2

4.01 - 8.0

Cotton underwear, FR pants and LS shirt, hard hat, arc rated face shield or flash hood, leather gloves and shoes, hearing protection

3

8.01 - 25.0

Cotton underwear, FR pants and LS shirt plus FR coverall, hard hat, arc rated flash hood, leather gloves and shoes, hearing protection

4

25.01 - 40.0

Cotton underwear, FR pants and LS shirt plus multi-layer flash suit, hard hat, arc rated flash hood, leather gloves and shoes, hearing protection

Thermography Inspection Practices

Electrical IR Practices Prior to NFPA 70e: Infrared cameras have been used to identify problems in electrical systems for many years. Problems in electrical systems manifest themselves by heating. An infrared camera can readily identify these problems in a thermal image,which provides an excellent method of identifying failing or problem components prior to a failure. A failure can disable an electrical system and cause significant lost production, equipment damage and bodily injury.

Infrared electrical inspection has been used by the insurance companies to help determine insurability and rates for industry for many years. More recently,users of infrared have found that they can use IR to prevent and predict failures to help further reduce down time equipment failure and increase overall safety.

Infrared cameras are based on digital camera technology and require a direct line of site to record an accurate image. In most cases, surveys are hampered by cabinet designs that obscure the target components being imaged and thermographers are put at risk by having to open cabinets or doors in an attempt to gain access to the internal components that they wish to image.

IR surveys of electrical systems are most valuable when the system is under heavy if not peak electrical load, which requires the thermographer to perform the inspection in and around live electrical components.

Typically, electrical system covers will be bolted in place and are removed while the inspection is underway and then replaced.

This working method comes in conflict with the requirements of NFPA70

Recommendations of NFPA70e as they relate to IR
One way that NFPA 70 E determines hazard and risk is based on the activity that you are conducting around the equipment. The scale is from zero to 4, where 4 has the highest risk potential. For example, removal of a bolted cover on 600-volt equipment carries a hazard/risk classification of 3 and that goes up to a 4 on anything greater than 600 volts.

As this work occurs within the flash protection boundary, the appropriate PPE must be worn. The required minimum PPE for Hazard/Risk Classification 3 work is to withstand 104.6 J/cm and the required minimum PPE for Hazard/Risk Classification 4 work is to withstand 167.36 J/cm. As much of the work performed for an IR inspection requires removal of bolted covers, this is the PPE that is required.

Additionally, NFPA70e recommends that only "qualified" personnel be allowed to perform work inside the flash protection boundary. A thermographer must be accompanied by a "qualified" person to remove covers. Both the thermographer and the additional person should be in full PPE.

Infrared Windows - Eliminate the Controllable Risk
The first rule in any risk assessment is to eliminate the risk if possible; PPE is always a last resort! Infrared windows eliminate the risks associated with live inspections as they allow an infrared camera direct line-of-site access to live electrical components without opening an electrical enclosure.

As such, they provide an excellent means of accessing electrical equipment efficiently and safely as a second qualified person is not required to open and unbolt enclosures and the "triggers" of arc flash are not introduced as the panels remain closed.

An IR window sounds more complicated than it really is, and although there are several types of window available on the market today, there is nothing stopping the thermographer from designing a window for use in any particular inspection that they may wish to complete.

An IR viewing window is basically an optic material that allows IR energy to pass through it and a holder/body; thermographers may even decide not to use a crystal if the energized component that you are interested in is some distance from the cover and a protective grill can be used in place of the crystal.

You must, however, ensure that the grill is IP2X certified – that is, the grill size must offer protection against foreign objects with diameters larger than 12 millimeters.

This method can significantly reduce the capital expenditure required and also has the additional benefits of allowing ultrasound inspections of the electrical switchgear as well as thermographic inspections. However, when using grills, operators will be exposed to live electrical components and as such will be required to wear the appropriate level of PPE identified from the arc flash hazard analysis of the switchgear.

The optics holder design depends upon a number of parameters; the field of view, equipment lens and window size are all functions of the design and must meet all the parameters that the thermographer requires before a holder is manufactured. Also, a protective cover should be included in the design as crystals are very expensive and, in some cases, extremely fragile.

An infrared window allows a thermographer to inspect the inside of the electrical enclosure without having to open or unbolt an enclosure door. As discussed earlier in this paper, the vast majority of arc incidents are triggered due to accidental contact, items dropping in and a change of state (introduction of air, dirt, moisture).

By keeping the door closed and sealed by inspecting with an infrared window, you have eliminated the controllable items that induce arc flash.

Infrared windows are available in multiple sizes and can be custom-made to retrofit dead fronts on distribution and isolator boards. The larger the size of the window, the greater the field of view one can see with the IR camera.

IRISS Infrared Windows

Where do I locate Infrared Windows?
To correctly install infrared windows, the targets that require inspection must be identified. Traditional surveys commonly only look at the bolted connections within the switchgear, as these are generally considered to be the "weakest points" or "points most likely to fail." These may include:

Once a decision has been made on what you require to see through your IR window, you will need to decide on what size of IR window you require, and where you need to install it to ensure maximum coverage and therefore maximum efficiency.

The formula for calculating the field of view (FOV) through an IR Window is:

2 x the tangent of one-half the angle x distance


Typically, infrared cameras have a standard FOV of approx 20 to 25 degrees in the horizontal, and 15 to 20 degrees in the vertical. Optional wide-angle lenses are typically two times the field of view, or about 50 degrees horizontal.

Another important consideration is to manipulate the camera when looking through an IR window. This has the effect of increasing what can be seen by up to a factor of three. This means that if your target is 12 inches across, you can reduce this to 4 inches (for IR window size calculation purposes) to allow for the additional area that will be seen when you manipulate the camera from left to right or up and down.

Once you have identified your required component identification and FOV calculations, you will be able to identify fairly accurately the number and size of IR windows that you require.

Electrophysics HotShot ECAB Solution

What Can I See Through an Infrared Window?
 An infrared window allows you to inspect the inside of an electrical cabinet to check the physical condition of the components that you have chosen to inspect. As with traditional thermographic inspections, we can see temperature differences very clearly. However, when trying to survey components that do not have any faults, very little load and are at the same temperature, you will see very little if anything at all.
 


Figure 6. Images taken through IR windows showing no apparent faults


Figure 7. Images taken through IR windows showing issues due to load imbalance, connections, etc.

You need to have the confidence in the infrared windows that you are using. They are designed to allow infrared energy to transmit through them at a known transmission rate. Therefore, if there is even a slight temperature difference, you will be able to see that with your IR camera, and be able to record images for the IR inspection trend analysis program.

How do I Use Infrared Windows?
An important thing to remember when using IR windows is to identify the window with a unique number. This will be invaluable, especially when you have multiple windows on electrical panels, etc. It is also advisable to identify the type and wavelength of the infrared window material.

The most essential data to record is the transmission rate of the crystal and also the emissivity of the component or components that you are measuring through the IR window. The most effective way of using IR windows is to (as discussed earlier) prepare all components that are inspected so as they have the same emissivity with electrical tape, paint, IR-ID labels, etc Thus, all components being inspected will have the same transmission rate and emissivity readings; consequently, the results gathered will be far more accurate.

How Do I Install Infrared Windows?
Installing an infrared window means cutting holes into very expensive switchgear, and before we complete this exercise, we need to be very sure that we are installing them in the correct place and that the switchgear ratings are not degraded in any way. This is not as daunting as it may sound. You should have already worked out how many windows you need, what IR windows you intend to use and where you want to put them. However, before we start the installation, we need to be sure of the following:

Rated
Maximum Voltage

Clearance

kV

In

Cm

4.76

5.5

14

8.25

6.5

17

15.0

8.0

20

27.0

12.0

30

38.0

15.0

36

Can IR Windows Carry a Generic Arc Rating?
Electrical switchgear comes in infinite shapes and sizes, and as such the surface areas and volumetric elements of the cabinets are different with each model, type and rating. Each cabinet is subject to the testing that is laid down by the certification bodies such as UL, IEEE, etc. This test is completed on the cabinet assemblies. When the testing is completed, the compliance is awarded to the assembly and not the components that make up the assembly.

A simple way of viewing this is to calculate the force that would be experienced on the surface of an electrical cabinet while undergoing an arc flash explosion test. The pressure exerted on a surface by a given force is determined by using the area over which that force acts. The formula to be used is:


Note:
For the purposes of these calculations, we are assuming that an arc flash explosion causes a uniform increase in pressure across the entire inside area of the chamber. However, you should note that pressure also increases with temperature, and so in an explosion where temperature increases, this would make these cases even more extreme, especially in smaller chambers.

Therefore, for a given force x, the pressure is inversely proportional to the area and hence as the area over which the force acts decreases the pressure will increase. This relationship can be seen in the graph above. This shows that if the force or explosion remains constant but the chamber and hence the area gets smaller, the pressure on each part of the chamber goes up according to the relationship shown.

Electrical cabinet designs and dimensions are infinite. We, therefore, CANNOT or MUST NOT use the data from one cabinet design to another design unless they are identical in every way.

This is the reason why components can never carry a generic arc rating and must be subjected to industry standard tests to confirm that they conform to the minimum required level of mechanical strength and environmental properties for the electrical cabinets and assemblies which they are going to be fitted into.

About this article

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