Session Three - ICEweb Safety/Evaluation of Arc... · Session Three: Evaluation of Arc Flash...

Session Three: Evaluation of Arc Flash Hazards Using Computer Based Simulation

2013 Electrical Arc Flash Conference - IDC Technologies

Session Three:

Evaluation of Arc Flash Hazards Using Computer Based

Simulation

Hilton Bennie

Senior Electrical Engineer, Applied Energy Solutions (Pty) Ltd

1. Introduction

Arc Flash Hazard Analysis is essentially the quantification of the thermal energy that

personnel might be exposed to during an arcing fault. The thermal energy released

during an arcing fault is proportional to current flowing during the fault as well as the

duration for which that current is sustained.

The duration for which an arcing fault is sustained is determined by the response of

the protective devices in the system – fuses, relays, etc. These are current

responsive devices, responding to a current flowing through an Arc Plasma – by

itself a complex phenomenon, dependent on many factors including the available

short circuit current, the pre-fault voltage, the gap between the conductors, system

pre-fault loading and configuration, etc.

Not only are the Arcing Current and Fault Clearing Times (FCT) complex to

determine, and dependant on many variables (some of which are out of our control

or difficult to predict), but the equations governing the incident energy are similarly

complex, based on statistical and empirical data.

As a result, determining the Arcing Current and FCT accurately can be challenging.

Even once these values are determined it is still difficult to intuitively gauge whether

one particular current/time combination is more or less hazardous than another,

without performing the full analysis.

As engineers are responsible for the safety of personnel, it is our responsibility to

perform the Arc Flash Hazard Analysis as accurately, yet conservatively, as

possible. Being overly conservative may yield unnecessarily strict PPE requirements,

which may be cumbersome and ultimately counter-productive.

Engineers need to have tools and systems in place to ensure a high level of

confidence in the results of the Arc Flash Analysis. We need the ability to perform

our analyses on various scenarios, making sure that we account for the worst

reasonable case.



2. Arc Flash – Is there really a threat?

All of my switchgear is Arc Proof, Metal Clad switchgear – is there still an Arc Flash

Hazard?

Although Arc Proof switchgear is designed to minimise the probability of internal

faults occurring (assuming that the manufacturer’s installation, operation and

maintenance guidelines were strictly adhered to), the possibility cannot be ruled out

entirely.

Assuming an internal fault does occur, however unlikely, the arc-venting capability of

the switchgear then comes into question. Some of the arc-venting capabilities

depend on the switchgear operation and maintenance. To determine whether or not

Arc Proof switchgear will provide adequate protection when required, many factors

need to be taken into account, for example:

1. Are the doors and front panels closed and secured correctly? Are

compartment doors open because work is being performed?

2. Are the pressure relief flaps in good working order and adequately secured?

3. Are the air ventilation closing flaps in good working order?

4. Was the switchgear originally specified correctly for the potential arcing fault

current? Has the arcing fault changed due to network changes?

There are many variables, and as engineers we cannot afford to be complacent. We

need to protect personnel against feasible threats.

3. IEEE 1584 – Arc Flash Hazard Analysis

A brief overview of the steps involved in performing Arc Flash Hazard calculations in

accordance with the IEEE 1584 standards is provided below.

3.1. Calculate the Arcing Current

The IEEE 1584 equations provide estimates for the Arcing current as follows:

System Voltage < 1000V:

System Voltage >= 1000V:

3.2. Calculate the Incident Energy

Once the Arcing current has been determined, the Incident Energy is calculated.

Incident Energy is the amount of energy impressed on a surface, at a certain

distance from the energy source. This energy is calculated in two steps – first the



normalised incident energy (En) is calculated, and then the actual incident energy

(E) is calculated from En. These equations are provided below.

Empirical Method – 1-15 kV

En is the energy incident on a surface at 24 inches (610 mm) with a fault clearing

time of 0.2 s.

E is the adjustment of En based on specific conditions.

Lee Method – 15+ kV



The incident energy is calculated using two different values of Arcing Current –

100% and 85%. The 15% reduction factor is in place to account for unknowns and

variations within the system. Since most of the current responsive devices in a

network would exhibit non-linear inverse-time characteristic responses, the FCT

would be increased. For each Arcing Current and corresponding FCT, the incident

energy is calculated, and the Arc Flash Hazard is assessed based on the highest

energy.

ETAP provides the user with the ability to override the default 15% arc current

variation with any value between 0-30%. As an illustration of the sensitivity of the

Incident Energy to this variation, please see the case study provided at the end.

In ETAP, the Empirical Method is used for voltage ranges between 0.208-15kV and

bolted fault currents in the range of 0.7-106kA. If the voltage and/or fault current

does not fall within this range, then the Lee Method is used.

3.3. Determine the Arc Flash Boundary

The flash boundary is defined as the distance from the arcing fault at which the

incident energy is equal to 1.2 cal/cm2.

Empirical Method – 1-15 kV

Lee Method – 15+ kV



3.4. IEEE 1584-2002 vs. NFPA 70E-2000

The NFPA and IEEE standards for Arc Flash Analysis provide very different results

under certain circumstances. The table below highlights some of the key differences

between capabilities of each standard.

Examining the difference between the calculated open-air Incident Energies at

various voltage levels and various Fault Clearing Times at a fixed working distance

of 18” (457mm), we see that the NFPA results are more conservative below 480V,

and less conservative above 480V. At 480V, both studies produce very similar

results. This is illustrated in the graphs below.



The graph above illustrates the more conservative results of the NFPA calculations

at 208V.

The graph above illustrates the more conservative results from the IEEE 1584

calculations at 600V.



Both the IEEE and NFPA calculation methods produce similar results for a 480V

system, as shown in the graph above.

There are various Arc Flash Calculation standards and methodologies available

beyond the IEEE 1584 and NFPA standards, and even the published standards

evolve over time. It is therefore important for engineers to remain informed of the

latest industry trends, and to really understand the standards that they are applying.

In particular, care should be taken to understand the limitations of the published

standards, and to carefully examine the applicability of a standard to a given

scenario.



4. Arc Flash Calculation Examples

To illustrate some of the key factors that influence the outcome of an Arc Flash

Hazard analysis, several examples are presented as case studies. Various sections

of an actual ETAP model developed for a local petroleum refinery will be used as the

basis for the case studies.

Some of the original data has been modified and/or simplified for purposes of

illustration.

4.1. Example 1

4.1.1. Introduction

The image below shows the section of the network used for this demonstration – it is

the left hand side bus of the LV MCC called SUB A. The bus is fed via a 750kVA

3.3/0.415 kV transformer and a 20m long 3 core 300mm2 copper XLPE cable.

The purpose of this example is to illustrate various influences on the Arc Flash

Hazard. Firstly, the difference between the arcing current and the bolted three phase

fault level is discussed. This is followed by an Arc Flash Hazard analysis of the

network under three Scenarios – with bus zone protection, without bus zone

protection and without protective device isolation.



4.1.2. Arcing current vs. Bolted fault current

The image below shows the results of a basic 3-phase short circuit calculation for a

fault on SUB A L bus. This value is calculated from the IEC 60909 standard.

The total 3-phase Bolted Fault Current on the SUB A L is 19.6kA, with 14.7kA

coming from the T-4 branch. These values correspond to the initial symmetrical RMS

short circuit current, or , as per the IEC 60909 standard.

The accuracy of an Arc Flash Analysis is heavily dependent on the accuracy of the

Arcing Current available, which is derived from the 3-phase bolted fault current.

Therefore, the Arc Flash accuracy depends on the accuracy of the fault calculation.

The fault current on the secondary side of a transformer depends on the fault level

on the primary side. If the primary is connected to a utility bus, the strength of the

network becomes the dominant factor in the available fault current (the fault current

is limited by the short circuit impedance of the network). The impact of the utility

impedance is illustrated in the graph below.



As the available primary side short circuit current increases (i.e. utility impedance

decreases), the available secondary side short circuit current approaches the

theoretical maximum.

If the network information is unknown, the maximum potential bolted fault current

(the magenta line on the graph above) is often used. While this assumption may be

adequate for component rating, it can be highly inaccurate (the weaker the network

the poorer the assumption). This is particularly noticeable during protection

coordination studies and arc flash analyses.

When no fault current information is available from the utility, the arc-flash hazard

analysis should include multiple scenarios, such as 50%, 75% and 90% of the

maximum 3-phase bolted fault current, and the worst-case scenario for the arc flash

hazard considered.

When calculating the arcing current, ETAP uses the total value and the formulae

from the IEEE 1584 standard to calculate the total arcing current ( ). As shown

below, this value is 10.31kA.



In order to calculate the contributions to the arcing current from each branch, the

ratio of each branch’s contribution to the total value is maintained. Therefore, the

contribution from the T4 branch is:

The Arcing Current contributions are significantly lower than the 3-phase bolted fault

currents. As an illustration of the effect that this reduced current magnitude may

have on the operation of a protective device, the image below shows the response of

the upstream relay, OC S1-SA FP1, to a phase current of 7.73kA vs. 14.7kA.



There is an additional 300ms delay in the response to the reduced current. This can

have a significant impact on the magnitude of energy available during the Arcing

fault.

4.1.3. Arc Flash Hazard Analysis

4.1.3.1. Scenario 1 – Bus Zone Protection

In this scenario, there is a Bus Zone protection scheme in place (provided by

DIFF_RELAY), and the main protective device (S1-SA IB2) is sufficiently isolated

from the Arc Flash such that the device is able to clear the fault.

From the results below, we see the total FCT is found to be 0.05s – this is 0.02s for

the differential relay to operate and 0.03s for the circuit breaker to trip. The working

distance is set to 18 inches (45.72cm).



The Arc Flash Hazard is identified as a Level 1 in this scenario.

This example illustrates the capability of a bus differential system to provide fast fault

clearing, potentially reducing the Arc Flash Hazard. The advantage of such a system

is that it is relatively easy to integrate into existing installations, but it can be

expensive due to the large number of CTs required.

4.1.3.2. Scenario 2 – Overcurrent Protection

In this scenario, the Bus Zone protection scheme is not in place, and the main

protective device (S1-SA IB2) is sufficiently isolated from the Arc Flash such that the

device is able to clear the fault.



In this scenario the fault is cleared by the protection relay on the incomer. The Arc

Flash Hazard is now identified as a Level 2.

4.1.3.3. Scenario 3

In this scenario, it is assumed that the main protective device is insufficiently isolated

from the arc flash, rendering the device incapable of clearing the fault. Clearing the

fault is therefore left to the upstream protective device.

The presence of the bus zone protection is irrelevant for the purposes of this

scenario as operation of the protective device is not possible. The image below

shows the modification of the SUB A L bus properties to ensure the Main Protective

device is not isolated.



Two important factors play a role in determining the FCT reported in this scenario.

Firstly, since the Source Protective Device is not sufficiently isolated from the fault, it

is assumed that S1-SA IB2 is unable to clear the fault. The fault is cleared by the

upstream breaker. Secondly, the Arcing Current used in this calculation is 85% of the

calculated arcing current, since this provides the worst-case incident energy. The Arc

Flash Hazard is now identified as a Level 4.

4.1.3.4. Load Protective Device vs. Source Protective Device

ETAP allows us to view the Arc Flash results for a fault located on the line side of

either a load or a source protective device simultaneously. Source protective devices

are determined automatically, and a fault on a source protective device implies that

the device is incapable of clearing the fault.

As an illustration, we repeat Scenario 02, but we adjust the Display Options to show

both Load and Source Protective devices (as shown in the image below).

The results are now displayed as follows.



We see that a fault on one of the Load PDs produces a Level 2 Hazard (fault is

cleared by the incoming breaker), whereas a fault on the line side of the incoming

breaker is cleared by the upstream breaker, producing a Level 4 Hazard.

4.2. Example 2 – LV Arcing Current Variation

This example illustrates the sensitivity of the incident energy to the arcing current

variation. As discussed previously, this variation is typically taken as 15% according

to IEEE 1584. In ETAP, this is user-definable between 0-30%, with 15% as the

default value. This variation only applies to LV systems.

This example uses the left hand side bus of a low voltage (415V) substation called

SUB O. This bus is fed via a 1.5MVA 6.6/0.415 kV transformer, and feeds two

MCCs.

4.2.1. Scenario 1

In this scenario, the Arcing current variation is set to 15% as shown below. This is

the default value, and the value suggested by the IEE 1584 standard.

ETAP will simulate an Arcing current of 100% and 85% of the calculated arcing

current, evaluate the worst-case scenario and report this.



The image above shows a section of the ETAP study case editor where the Arc

Current Variation is set for LV systems.

The image above clearly indicates that the 85% Arcing Current provides the worst-

case incident energy, and the Arc Flash Hazard is identified as Level 3.

4.2.2. Scenario 2

In this scenario, the Arcing current variation is set to 30%, meaning that ETAP will

simulate with 100% and 70% of the calculated Arcing current and report the worst-

case incident energy.



The image above shows the modification to the study case editor.

Again, the lower Arcing Current provides the worst-case scenario, but this time the

Arc Flash Hazard is reported as Level 4.

4.3. Example 3 – Network Configuration

This example illustrates the sensitivity of the incident energy to the network

configuration. In this case, we examine the medium voltage substation called SUB 7,

which consists of two buses fed via 2x 12.5MVA transformers, separated by a Bus

Section breaker. An arcing fault on the right hand side bus is induced.



4.3.1. Scenario 1 – Normal Configuration

This scenario represents the normal operating conditions of the network. Both

transformers are in service and the Bus Section breaker is open. An Arc Flash on

one of the busses produces the following results.

The Arc Flash Hazard on SUB 7 R is indicated as Level 1 for this network

configuration.

4.3.2. Scenario 2 – Expected Worst-Case

This scenario represents the expected worst-case operating conditions. One of the

incoming transformers is out of service, and therefore the Bus Section breaker is

closed. An Arc Flash on one of the busses now produces the following results.



The Arc Flash Hazard is now identified as Level 2. This is indeed worse than

Scenario 1, as anticipated.

4.3.3. Scenario 3 – Unexpected Worst-Case

This scenario represents the actual worst-case operating conditions. Both incoming

transformers are in service, however the Bus Section breaker has been closed.

Although this is an undesired network configuration, it is not impossible that this can

occur. A lack of respect for operating procedures, inadequate training of personnel,

and failure/tampering with safety interlocks could lead to such a scenario occurring.

An Arc Flash on one of the busses now produces the following results.



We now have a Level 3 hazard. Unless adequate measures are in place to prevent

such a condition from occurring, the feeder panels from this substation would need

to be rated as Level 3.



4.4. Example 4 – Maintenance Mode

4.4.1. Introduction

ETAP gives you the ability to simulate the operation of maintenance mode switches.

Newer solid state trip units are being equipped by the manufacturer with special

maintenance mode switches which allow the override of the normal settings to use

instead very fast operation settings with very low pickup values. The purpose of

these trip units is to override normal coordination settings while energized work is

being performed in order to minimize the fault clearing time.

An example would be the Eaton ARMS (Arc Reduction Maintenance System). The

ARMS provides an accelerated instantaneous trip to reduce arc flash hazards – trip

times as low as 18ms are claimed. This feature is available on certain devices within

the Magnum and NRX range.

There are 5 levels of protection to facilitate the maximum arc flash reduction while

avoiding nuisance tripping during planned start-up and maintenance operations

without disturbing the normal operational trip unit settings.

The results of testing the ARMS system are illustrated in the graph below. Using the

ARMS Maintenance Mode, the Incident Energy is kept below a Category 1 for fault

currents between 10kA and 100kA.



4.4.2. ETAP Simulation

We illustrate the possibility of using ETAP to simulate the operation of the same

protective device operating in either normal mode or maintenance mode using a

small section of the LV substation called SUB M.

We utilise a feature of ETAP’s 3-Dimensional database, called Revisions, to create

two versions of the engineering data.

In the Base revision (indicated by the 3-D Database Toolbar on the right of the image

below), we have the trip device operating normally. The Arc Flash Hazard analysis is

shown below.



We are sitting with a Level 3 hazard on the feeder panels. However when energised

work is going to be performed, the solid state trip device associated with the

incoming breaker (S4-SM IB2) may be switched over into Maintenance Mode.

To simulate this, we create a new ETAP revision, called Maint Mode, and set the LV

breaker trip device to maintenance mode within this revision, as shown below.



A new Arc Flash Analysis calculation is run, yielding the following results.



We see that the Hazard has been reduced from Level 3 to Level 1 by reducing the

FCT from 0.21s to 0.04s. The difference between the Maintenance Mode and normal

operation is demonstrated clearly by the TCC curve below.



5. Summary and Conclusion

Calculation of Arcing Current, Fault Clearing Time and Incident Energy is complex

and is best performed using computer-based simulation of multiple different

scenarios to allow engineers to make accurate, informed decisions regarding Arc

Flash Hazards.

It is the responsibility of engineers to specify switchgear correctly, to set protective

devices appropriately, to provide adequate warnings of potential hazards and to

ensure that personnel are provided the correct PPE for the tasks they are expected

to perform.

Arc Flash Hazards change over time as a result of several factors, including internal

network evolution (loads, generation assets, configuration changes, protection

coordination changes, etc.) as well as external (utility) network evolution. Anything

that alters the available fault current and the protection coordination can potentially

affect the Arc Flash Hazard.

Correct Arc Flash Analysis requires complete, accurate, up-to-date information, a

thorough understanding of the network under investigation, and a suitable

appreciation of the applicable standards and their limitations. It is important for

engineers to have the best tools at their disposal for performing the arc flash

analyses – tools that easily allow the engineer to perform calculations across

multiple scenarios quickly and compare the results easily. One such tool is ETAP.

6. Bibliography

1. UniGear ZVC - Internal Arc Containment, Resistance And Arc Flash Mitigation

- Supplement brief to 1VGA672001 – Rev F

ABB Australia Pty Ltd, January 2010.

2. Design of a New Generation of Internal Arc Resistant Switchgear

AREVA Technology Centre

Nirmal Deb, Patrick Bailly, Thierry Tricot, Leslie T Falkingham.

3. OPTIONS FOR MANAGING SAFETY WITH ELDERLY SWITCHGEAR

RPS Switchgear Limited

Richard Blakeley, 2010

4. What is the comparison of NFPA 70E-2000 an IEEE 1584-2002 standards for

Arc Flash Analysis?

ETAP FAQ # 15

Operation Technology, Inc., June 2003

5. ETAP 12.0 User Guide

Operation Technology, Inc., January 2013

6. ARC FLASH HAZARD ANALYSIS AND MITIGATION

Christopher Inshaw, Robert A. Wilson



Western Protective Relay Conference, October 2004

7. Evaluating the Hazards of Low-Voltage Arcs

Albert Marroquin, Operation Technology Inc

Published in Electrical Products & Solutions, June 2007

8. IMPACT OF AVAILABLE FAULT CURRENT VARIATIONS ON ARC-FLASH

CALCULATIONS

IEEE Paper No. PCIC-2009-16

Ilanchezhian Balasubramanian, Aidan M. Graham

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