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Home / Technical Articles / Switchgear interlocking system and arc protection that you MUST consider in the design

Safety in the first place

There are at least two safety requirements that medium-voltage switchgear MUST fulfil: an interlocking system and an arc protection system. Yes, these two systems are crucial in terms of safety because they protect not only the operator and other substation personnel but also the equipment in the substation itself.

Switchgear interlocking system and arc protection that you MUST consider in the design
Switchgear interlocking system and arc protection that you MUST consider in the design

There are hundreds and hundreds of incidents worldwide that usually involve wrong breaker manipulation that leads to catastrophic consequences and injuries.

This article sheds some light on the most common interlocking and arc protection systems installed in a medium voltage switchgear.

Table of Contents:

  1. Switchgear interlocking systems
    1. Interlocking methods:
      1. Interlock scheme #1: Two incomers and bus coupler interlocking
        1. Electrical interlocking
        2. Mechanical interlocking
      2. Interlock scheme #2: Incomer circuit breaker and earth switch interlocking
      3. Interlock scheme #3: Feeder circuit breaker and earth switch interlocking
  2. Internal Arc Classification (IAC)
    1. Relevant standards and testing
      1. IAC certification example
    2. Causes of internal arc
    3. Minimizing the effects

1. Switchgear Interlocking Systems

Interlocking between different switchgear apparatus and enclosure access covers and doors enhances personnel safety, as well as improving operational convenience. If a switching device can cause serious damage in an incorrect position, this must also have a locking facility.

Interlocks consists of the rules. Interlocks MUST ensure that the disconnector cannot be moved or operated unless the circuit breaker is open. Interlocks shall ensure that the circuit breaker cannot be closed unless the disconnector is fully in the ‘closed‘, ‘isolated‘ or ‘earth‘ position.

Interlocking uses electrical and mechanical methods or a combination of both. IEC 62271-200 states mandatory rules for switchgear interlocking:

For metal-enclosed switchgear with removable switching apparatus:

Rule #1 – Switching device must be in the open position before it can be withdrawn.

Rule #2 – Switching device can only be operated in the positive service or test position.

Rule #3 – Switching device cannot be closed unless the auxiliary control circuits required to open the switch are connected. Auxiliary control circuits cannot be disconnected with the switching device closed in the service position.

For metal-enclosed switchgear with disconnectors:

Rule #1 – Disconnector cannot be operated under conditions other than those for which it is intended to be used.

Rule #2 – Disconnector cannot be operated unless the main switching device is open.

Rule #3 – Operation of a main switching device is prevented unless its associated disconnector is in a positive service, test or earth position.

Rule #4 – Disconnectors providing isolation for maintenance and servicing must have a locking facility.

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1.1 Interlocking Methods

The illustration in Figure 1 shows a common switchgear arrangement for a medium voltage power distribution system. This switchgear arrangement uses three separate interlocking methods which are marked with 1, 2 and 3 in the red squares.

Let ‘s now start with describing of each scheme.

Figure 1 – Typical MV power distribution switchgear arrangement with interlocks

Typical MV power distribution switchgear arrangement with interlocks
Figure 1 – Typical MV power distribution switchgear arrangement with interlocks

Where:

  • Q-IL – Circuit breaker (left incomer)
  • E-IL – Earth switch (left incomer)
  • TXR_L – Supply transformer (left bus)
  • Q-IR – Circuit breaker (right incomer)
  • E-IR – Earth switch (right incomer)
  • TXR_R – Supply transformer (right bus)
  • Q-BC – Circuit breaker (bus coupler)
  • Q-FL – Circuit breaker (left feeder)
  • E-FL – Earth switch (left feeder)
  • Q-FR – Circuit breaker (right feeder)
  • E-FR – Earth switch (right feeder)

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Interlock Scheme #1

Two Incomers and Bus Coupler Interlocking

The two incomers and the bus coupler circuit breakers use a standard “2 out of 3” interlocking system to prevent a parallel feed from the two incomers onto a common bus. Interlocking allows the following conditions:

Condition #1 – The two incomer circuit breakers closed (Q-1L and Q-1R) with the bus coupler circuit breaker open (Q-BC).

Condition #2 – Left incomer and bus coupler circuit breakers closed (Q-IL and Q-BC) with right incomer circuit breaker open (Q-IR).

Condition #3 – Right incomer and bus coupler circuit breakers closed (Q-IR and Q-BC) with left incomer circuit breaker open (Q-IL).

Typically, these interlocking conditions are met using both electrical  and mechanical method.

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Electrical Interlocking

Normally closed auxiliary contacts from the two incomers and the bus coupler circuit breakers are used to electrically interlock the close command of each circuit breaker.

Left incomer circuit breaker (Q-IL) has a normally closed auxiliary contact from the right incomer circuit breaker (Q-IR) and a normally closed contact from the bus coupler circuit breaker (Q-BC) connected in parallel to allow a close command.

Right incomer circuit breaker (Q-IR) has a normally closed auxiliary contact from the left incomer circuit breaker (Q-IL) and a normally closed contact from the bus coupler circuit breaker (Q-BC) connected in parallel to allow a close command.

Bus coupler circuit breaker (Q-BC) has a normally closed auxiliary contact from the left incomer circuit breaker (Q-IL) and a normally closed contact from the right incomer circuit breaker (Q-IR) connected in parallel to allow a close command.

This control method only allows for any two circuit breakers to be closed at the same time.

Figure 2 – Interlock scheme #1: Two incomers and bus coupler interlocking

Interlock scheme #1: Two incomers and bus coupler interlocking
Figure 2 – Interlock scheme #1: Two incomers and bus coupler interlocking

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Mechanical Interlocking

Mechanical locks must be used to prevent unauthorized access to areas of the switchgear panel that contain live components. Unless all live parts have been rendered safe – either by a clearly applied earth connection or by being positively disconnected and screened from the other live parts – access to such parts is not possible.

To guarantee effective and comprehensive protection against malfunction, mechanical interlocks must be supplied. Mechanical interlocks must be built and engineered to ensure reliable fail-safe performance.

In our case, interlocking uses a key system which includes three identical locks and two identical keys. Both incomer and the bus coupler circuit breakers (Q-IL, Q-IR, Q-BC) require an interlock key to be inserted into the circuit breaker body, and the circuit breaker racked into the service position, before the circuit breaker can be closed. This interlock key can only be removed when the circuit breaker is open and in the racked-out test position.

When the circuit breaker is in service, the interlock key is not accessible.

Both incomer and the bus coupler circuit breakers (Q-IL, Q-IR, Q-BC) are fitted with identical locks but only two matching keys are available. Under normal operating conditions, the two incomer circuit breakers are closed using the two available interlock keys. The bus coupler circuit breaker is not permitted to close.

If one of the incomer supplies is lost, the associated circuit breaker is opened and racked-out to the test position. The interlock key can be moved to the bus coupler circuit breaker, allowing it to be racked into the service position and closed. When normal supply resumes, the bus coupler circuit breaker has to be opened before the revived incomer circuit breaker can be closed using the interlock key retrieved from the bus coupler circuit breaker.

This key interlock system only allows for any two circuit breakers to be closed at the same time.

Further Study – Learn how to interpret interlocking schemes between MV cubicles

Learn how to interpret interlocking schemes between MV cubicles (single line and wiring diagrams)

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Interlock Scheme #2

Incomer Circuit Breaker and Earth Switch Interlocking

The incomer circuit breaker (Q-IL or Q-IR) and earth switch (E-IL or E-IR) are mechanically interlocked to prevent both being closed at the same time. The earth switch can only be closed once the circuit breaker is open and racked-out to the test position. The circuit breaker can only be racked-in for closing, once the earth switch is open.

An additional level of interlocking is required. The incomer earth switch cannot be mechanically operated until power is removed from the incoming supply. This prevents closing the earth switch onto a live supply.

This interlocking is achieved in one of two ways:

Way #1 – Mechanically by using key access. The incomer earth switch (E-IL or E-IR) handle operation is only accessible by using a key, retrieved from the upstream circuit breaker when it is open and racked-out.

Way #2 – Electrically by using a solenoid. A solenoid is energized when the upstream circuit breaker is open and racked out, allowing access to the incomer earth switch (E-IL or E-IR) handle operation.

Figure 3 – Interlock scheme #2: Incomer circuit breaker and earth switch interlocking

Interlock scheme #2: Incomer circuit breaker and earth switch interlocking
Figure 3 – Interlock scheme #2: Incomer circuit breaker and earth switch interlocking

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Interlock Scheme #3

Feeder Circuit Breaker and Earth Switch Interlocking

The feeder circuit breaker (Q-FL or Q-FR) and earth switch (E-FL or E-FR) are mechanically interlocked to prevent both being closed at the same time. The earth switch can only be closed once the circuit breaker is open and racked-out to the test position. The circuit breaker can only be racked-in for closing, once the earth switch is open.

Figure 4 – Interlock scheme #3: Feeder circuit breaker and earth switch interlocking

Interlock scheme #3: Feeder circuit breaker and earth switch interlocking
Figure 4 – Interlock scheme #3: Feeder circuit breaker and earth switch interlocking

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2. Internal Arc Classification (IAC)

Metal enclosed switchgear can suffer internal faults at numerous locations, causing a wide range of physical damage. Internal Arc Classification (IAC) of metal enclosed switchgear considers the damage that can affect covers, doors, inspection windows, ventilation openings etc., as a result of overpressure within panel compartments.

IAC also takes into consideration damage from thermal effects, ejected hot gases and molten particles. When selecting metal enclosed switchgear, the probability of internal arcing and the safety risk to operators and the general public needs to be considered. Where the safety risk is considered relevant, the switchgear should be IAC classified.

The IAC classification indicates the maximum fault current level and duration to which the switchgear has been tested. When choosing switchgear, the IAC rating should exceed the expected fault current level and duration at the point of installation.

The rating also takes into account the accessibility of the switchgear. IAC tested and certified switchgear must always be clearly marked with the classification, fault level and duration, and accessibility of each side.

Suggested Reading – The art of arc-protection relaying in MV applications

The art of arc-protection relaying in MV applications

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2.1 Relevant Standards and Testing

The primary standard for internal arc classification of medium voltage metal enclosed switchgear is IEC 62271-200; IEC 62271-202 is also relevant. IEC 62271-200 details test procedures to assess damage to switchgear from internal arcing. Test results provide the switchgear with an IAC classification. Switchgear which passes indoor testing is also considered suitable for outdoor use with the same accessibility requirements.

Accessibility is divided into two categories, Type A is for authorised personnel dressed with adequate protective equipment and Type B for general public access. Equipment is also tested for different directions of access: front, rear or lateral (side).

Cotton cloth indicator panels are placed 2m above ground level and on each accessible side of the equipment under test. If pressure relief ducts are part of the switchgear design, these must also be subjected to cloth indicator panel testing.

Tests are carried out by supplying a predetermined level of fault current for a specific duration. Using various test procedures, the applied fault current creates an internal arc to ground within a specific region of the switchgear.

In general, test results are considered acceptable if:

  1. Correctly secured doors and covers do not open – deformation is acceptable, providing it doesn’t protrude as far as the indicator panels
  2. No fragmentation of the enclosure occurs within the test time – small particles up to 60 g are acceptable
  3. Arcing does not cause any holes in the accessible areas, up to a height of 2 metres
  4. Indicator panels do not ignite due to hot gas emissions
  5. The enclosure remains connected to its earth point (verified by a continuity test)

Suggested Video – MCset internal Arc tests

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IAC Certification Example

Indoor room testing: The room is simulated by a floor, ceiling and two walls perpendicular to each other.

Accessibility:

  • Type A: Restricted to authorized personnel
  • Type B: Unrestricted accessibility
  • F = Front access
  • L = Lateral (side) access
  • R = Rear access

IAC certification example:

  • IAC classification: AF
  • Internal Arc: 31.5 kA, 1 s

Figure 5 – IAC certification

IEC 62271-200 and Internal Arc Classification (IAC)
Figure 5 – IEC 62271-200 and Internal Arc Classification (IAC)

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Causes of Internal Arc

There are many potential causes for internal arcing within metal enclosed switchgear. Some of the more common causes are:

  1. Foreign matter in the enclosure (eg vermin, metal swarf, tools)
  2. Contamination and general degradation of insulation material
  3. Inadequate insulation of cable terminations
  4. Overheating of termination points due to inadequate preparation and tightening
  5. System overvoltage
  6. Incorrect protection settings and coordination

Table 1 – Locations, causes and examples of measures to decrease the probability of internal faults

Locations where internal faults are most likely to occurPossible causes of internal faultsExamples of possible preventive measures
Cable compartmentsInadequate designSelection of adequate dimensions.
Use of appropriate materials.
Faulty installationAvoidance of crossed cables connections.
Checking of workmanship on site. Correct torque
Failure of solid or liquid insulation (defective or missing)Checking of workmanship and/or dielectric test on site.
Regular checking of liquid levels, where applicable
Disconnectors Switches Earthing switchesMaloperationInterlocks. Delayed reopening.
Independent manual operation. Making capacity for switches and earthing switches. Instructions to personnel.
Bolted connections and contactsCorrosionUse of corrosion inhibiting coating and/or greases. Use of plating. Encapsulation, where possible.
Faulty assemblyChecking of workmanship by suitable means. Correct torque. Adequate locking means.
Instrument transformersFerro-resonanceAvoidance of these electrical influences by suitable design of the circuit.
Short circuit on LV side for VTsAvoid short circuit by proper means for example, protection cover, LV fuses.
Circuit breakersInsufficient maintenanceRegular programmed maintenance. Instructions to personnel.
All locationsError by personnelLimitation of access by compartmentation. Insulation embedded live parts. Instructions to personnel.
Ageing under electric stressesPartial discharge routine tests.
Pollution, moisture, ingress of dust, vermin, etc.Measures to ensure that the specified service conditions are achieved. Use of gas-filled compartments.
OvervoltagesSurge protection. Adequate insulation co-ordination. Dielectric tests on site.

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Minimizing the Arcing Effects

Certain design techniques are used to provide a high level of safety to personnel, by minimizing the effects of internal arcing:

  1. Compartmenting of enclosure
  2. Pressure relief methods
  3. Double skin panels
  4. Arc venting away from access areas
  5. Remote control of switchgear
  6. Rapid fault clearance

Rapid fault clearance requires fast detection and isolation of the arc. This can be achieved using:

  1. light, heat or pressure sensors combined with a relay to trip a fast acting circuit breaker
  2. pressure operated earth switch capable of diverting the internal arc to ground (arc eliminator)
  3. fast acting, current limiting line supply fuses

Suggested Video – Internal arc distances

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Sources:

  1. MV application guide by Aucom
  2. 11 kV Indoor Switchgear – SCADA Controlled by NSW Government

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

4 Comments


  1. Adise Beyene
    Jun 30, 2023

    It’s useful to me, thanks so much


  2. JOSE MELO
    Mar 13, 2023

    Muito bom artigo de segurança para ser usado na Operação de subestações.


  3. Shehada
    Feb 01, 2023

    Such enclosure certified as per IEC 62271-200, are they tested against the worst case arc flash that can occur? I.e. even for category 4 (>40kcal/cm2)


  4. Arockia Das Middle Name Antony
    Jan 31, 2023

    Very useful-Thank you

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