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What is an Arc Flash Study? Two Official Methods Described.

According to NFPA 70E, it is a study that:

It as also referred to as an Arc Flash Risk Assessment.

NFPA 70E is an American standard that is managed by the National Fire Protection Association (NFPA). The full name is NFPA 70E, Standard for Electrical Safety in the Workplace

An Arc Flash Study probably sounds complicated.

And you may have only one LV switchboard in your plant. Or you may have a large industrial plant with many HV and LV switchboards.

You may wonder:

In this article, I explain why it is required, and I will describe the two official Arc Flash Study methods that you can use.

Let us start with the first question.

Why is an Arc Flash Study required?

It is a requirement of NFPA 70E, Standard for Electrical Safety in the Workplace.

In the NFPA 70E standard, an Arc Flash Study is referred to as an Arc Flash Risk Assessment.

NFPA 70E, in turn, refers to IEEE 1584 for when the Incident Energy Method is used. Note that IEEE 1584 uses the term Arc Flash Study.

NPFA 70E is not directly enforced from the OSH Act of 1970 in the USA, or the relevant federal regulations. However, it is considered by the OSH Act as an industry Consensus Standard.

Note that each state in the USA may have own specific requirements.

This same rule applies in most other countries, where there is no alternative standard for NFPA 70E.

In the absence of anything better, it best that you follow the NFPA 70E standard. Or at least the intent of the standard.

Lets just cover two basic definitions before we go into more detail.

What is arc flash incident energy?

The amount of thermal energy at a certain distance from an arc source. It is expressed in calories per square centimeter (cal/cm2).

What is an arc flash boundary?

The distance at which the incident energy equals 1.2 cal/cm2. At 1.2 cal/cm2 an operator is likely to receive second degree burns if arc rated PPE is not used.

Which Arc Flash Study method should I use?

There are two official methods defined by NFPA 70E:

Arc Flash Study Method 1: Incident Energy Analysis Method

The incident energy analysis method is defined by NFPA 70E, and IEEE 1584.

The incident energy and arc flash boundary for each circuit are calculated with the equations in IEEE 1584.

It is calculated from the parameters in the table below.

It is quite difficult to use the equations in IEEE 1584. It is easier to use an online arc flash calculator or modeling software.

Arc Flash Study -Incident Energy Method
Parameter Description
Nominal voltage The nominal operating voltage. For example, 400 V, 600 V or 15 kV.
Working distances You can determine this yourselves or use the typical working distances in NFPA 70E.
Short circuit current This is the maximum prospective bolted short circuit current. You can get this from an existing short-circuit study. If one is not available, the short-circuit currents will have to be calculated as part of the arc flash study.
Protection settings This is the combined clearing time of the protection relay, and the circuit breaker or the fuse clearing time.
Conductor spacing Determine the gaps between conductors, where the arc is likely to occur. You can get this from design documentation or measure it. If you cannot find this information you may use the typical information from IEEE 1584.
Enclosure sizing Determine the enclosure size, where the arc is likely to occur. You can get this from design documentation or measure it. If you cannot find this information you may use the typical information from IEEE 1584.
Electrode configuration IEEE 1584 defines 5 different types:
  • VCB: Vertical conductors inside a metal enclosure.
  • VCBB: Vertical conductors terminated in an insulating barrier inside a metal enclosure.
  • HCB: Horizontal conductors inside a metal enclosure.
  • VOA: Vertical conductors in open air.
  • HOA: Horizontal conductors in open air.

Select PPE for the Incident Energy Method

For the Incident Energy Analysis Method, the PPE is divided into the following two levels.
  1. Incident energy exposures equal to 1.2 cal/cm2 up to 12 cal/cm2
  2. Incident energy exposures greater than 12 cal/cm2

There are no categories, and the arc rating specified on the PPE must be higher than the calculated energy, which must be shown on the label.

Arc Flash Label for the Incident Energy Method.

When the Incident Energy Analysis Method is used, the label must include the following:

  1. System voltage.
  2. Arc flash boundary.
  3. Incident energy, or minimum arc rating of clothing.

Not PPE categories!

The sample label was created with https://www.mysafetylabels.com.

Arc Flash Label for the Incident Energy Method USA

Method 2: Arc Flash PPE Category Method.

Arc Flash Study-PPE Category Method

The arc flash PPE Category Method is defined by NFPA 70E.

It is a two-step process:

  1. Step 1: Determine the Likelihood of an arc flash.
  2. Step 2: Select the PPE Category and the Arc Flash Boundary.

Step 1: Determine the Likelihood of an arc flash.

Look up the relevant activity in NFPA 70E Table 130.5(C) to determine the likelihood of an arc flash.

The table below shows three examples.

Task Equipment condition Likelihood of arc flash
Reading a panel meter while operating a meter switch. Any No
Insertion or removal (racking) of circuit breakers (CBs) or starters from cubicles, doors open or closed. Any Yes
Operation of a CB, switch, contactor, or starter. Normal No.
I do not necessarily agree with this assessment.

If the likelihood is Yes, then additional protective measures must be implemented. For example:

  1. Install arc tested switchgear.
  2. Remote racking and switching.
  3. Provide arc rated PPE.

In this example, we will focus on PPE selection.

Step 2: Select the PPE Category and the Arc Flash Boundary.

In this step, you select the PPE category and arc flash boundary from a table with predefined parameters and categories.

The PPE is defined into 4 categories:

Category Minimum arc rating of PPE
1 4 cal/cm2
2 8 cal/cm2
3 25 cal/cm2
4 40 cal/cm2

Note that in many large industrial plants only category 2 and 4 are used.

You need the following parameters to select from the PPE Categories table:

Parameter Description
Nominal voltage The nominal operating voltage. For example, 400 V, 600 V or 15 kV.
Working distances You have to comply with the pre-defined working distances in NFPA 70E. You don't have to determine your own distances.
Short circuit current This is the maximum prospective bolted short circuit current. You can get this from an existing short-circuit study. If one is not available, the short-circuit currents will have to be calculated as part of the arc flash study.
Fault clearing time This is the combined clearing time of the protection relay, and the circuit breaker or the fuse clearing time. You can work this out from the bolted-fault currents and the protection settings.
Type of equipment For example, switchgear or batteries.
Condition of the switchboard Normal or Abnormal.

Normal means:
  1. The equipment is properly installed.
  2. The equipment is properly maintained.
  3. The equipment is used accoring to the manufacturer's instructions.
  4. Equipment doors are closed and secured.
  5. Equipment covers are in place and secured.
  6. There is no evidence of impending failure.

When you have the parameters, you select the PPE category and arc flash boundary from NFPA 70E in Table 130.7(C)(15)(a).

Here is one example.

Equipment PPE Category Arc Flash Boundary

Parameters:

  • Maximum available fault current: of 42 kA.
  • Maximum fault clearing time 0.33 sec.
  • Minimum working distance 455 mm (18 in.).
4 4.3 m

This means, for racking on a switchboard with a 42 kA fault current, clearing time of 0.33 seconds you need to use category 4 PPE, and the arc flash boundary is 4.3 meters.

Arc Flash Label for the PPE Category Method.

According to NFPA, the label for the arc flash PPE categories method must include the following:

  1. Nominal system voltage.
  2. Arc flash boundary.
  3. The PPE arc flash category, or the minimum arc rating of PPE.

Not the incident energy!

Arc Flash Label-PPE Category Method USA

Assess Arc Tested Switchboards with the PPE Category Method.

The PPE Category Method also applies to switchgear that has been tested for an internal arc.

NFPA 70E covers high voltage switchgear only.

Here is an example from NFPA 70E Table 130.7(C)(15)(a).

Equipment PPE Category Arc Flash Boundary
  • Arc-resistant switchgear, 1kV to 15 kV.
  • Protection clearing time of less than 0.5 seconds.
  • Available fault current less than arc-resistant rating.
  • All door and covers closed.
  • The condition is normal.
NA NA

According to the table above, arc rated PPE is not required.

However, this may not be the best decision. I will discuss this in another article.

Furthermore, when a manufacturer states that the switchboards are arc rated, there may still be potential issues. And you have to do the following:

  1. Obtain the arc flash certificates.
  2. Perform a detailed arc flash test certificate analysis.

Perform detailed arc flash test certificate analysis

During the analysis you have to answer the following questions:

  1. Is the prospective bolted fault current less than the current stated on the test certificate?
  2. Is the protection clearing time less than the time stated on the test certificate?
  3. To what standard have the switchgear been tested for internal arc?
  4. Was it an actual standard, or just a guideline?
  5. How old is the standard?
  6. Is the standard applicable you your country?
  7. Have you received the test certificates to confirm the rated arc current and time?
  8. Was the switchboard tested by a certified test laboratory?
  9. Did you install the same switchboard as the version that have been tested?
  10. Did they make any modifications to the tested switchboards that need to be applied to your switchboards?
  11. Was the configuration of the switchboard that they tested, the same as yours? For example, did they use fuses instead of circuit breakers?
  12. Did you install the switchboard according to the manufacturer's recommendation, including arc vents, etc.?
  13. Is the switchboard maintained properly?
  14. Did they test all the relevant compartments where an arc may develop?
  15. If not, how can you implement additional controls to mitigate the risk?

Internal arc testing of HV versus LV switchgear

There is a distinct difference between HV and LV switchgear internal arc testing.

There are well defined standards for testing HV switchgear for an internal arc.

And there are no official standards for LV switchgear for internal arc! There are only guidelines, and it is based on an agreement between the supplier and the manufacturer.

This means that they may have been tested on agreed compartments and agreed parts of the circuit only.

They may also have been tested with fuses, whilst your shipboard is equipped with circuit breakers.

For LV switchgear, you have to answer all the questions listed above, but pay special attention to:

You can even go a step further and request the arc flash test reports. This is a detailed proprietary test report. If allowed, you will most likely have to sign a non-disclosure agreement and view them at the manufacturer's premises.

The lack of standardization in LV switchgear arc testing may be the reason why the NFPA 70E covers only HV switchgear in the PPE Category method.

Do I need to use PPE if a switchboards is arc rated? What label should I use?

I will use the dreaded word, it depends. And I will cover this in another article.