What is Inside the Control Center of Medium Voltage Switchgear 

• May 21, 2026
• Gaurav Joshi

The low voltage compartment is often called the brain of medium voltage switchgear. Even the most expensive breaker becomes useless if something fails inside this section.

This compartment controls protection, indication, monitoring, and switching operations. Without it, the switchgear cannot operate properly.

Understanding the low voltage compartment becomes very important for engineers and maintenance staff. Once you understand what exists inside it, switchgear operation becomes much easier to understand.

Table of Contents

• Why It Is Called the Low Voltage Compartment
• Location of the Low Voltage Compartment in Switchgear
• Supply Sources for the Low Voltage Compartment
• Common Components Inside the Low Voltage Compartment
• Understanding an Actual Low Voltage Compartment
• Why the Low Voltage Compartment is So Important
• Conclusion

Why It Is Called the Low Voltage Compartment

Imagine a 36kV medium voltage switchgear panel. The cable compartment operates at 36kV. The circuit breaker chamber also works at 36kV. Similarly, the busbar compartment carries the same voltage level.

However, one compartment operates at much lower voltage levels. That section may use:

• 110V DC
• 220V DC
• 48V DC

This section is called the low voltage compartment because its operating voltage remains below 1000V. Some people also call it the metering and relay chamber.

The operating voltage may change depending on the project and customer requirement. However, the purpose remains the same.

The equipment inside this compartment cannot directly operate at 36kV. For example, building a 36kV relay or 36kV meter would make the system extremely large and impractical.

That is why engineers use instrument transformers. These transformers step down voltage and current to safe values. Then the low voltage devices can operate properly.

Location of the Low Voltage Compartment in Switchgear

The location of the low voltage compartment may vary from one design to another. In most medium voltage switchgear panels, engineers place it at the top section.

This arrangement allows operators to access relays, meters, and indication lamps easily.

Ring main units also place the low voltage devices at the upper section. In extra high voltage GIS systems, the compartment may contain HMIs, relays, and monitoring systems.

Some manufacturers even provide low voltage sections at both the top and bottom. There is no fixed design rule here. The design changes based on:

• Project requirements
• Customer preference
• Protection requirements
• Manufacturer design philosophy

That is why no two low voltage compartments always look identical.

Still, many common components appear in almost every design.

Supply Sources for the Low Voltage Compartment

The low voltage compartment requires multiple supply sources for operation. The first source is the auxiliary power supply. This supply may come from:

• Battery banks
• Auxiliary transformers
• DC supply systems

The auxiliary system feeds:

• Relays
• Indication lamps
• Annunciators
• Control circuits

Most systems commonly use 110V DC or 220V DC supply.

The second important source is the current transformer. Suppose the incoming feeder carries 800A current. The current transformer steps it down to 1A or 5A. That reduced current then feeds relays and meters safely.

The third source is the voltage transformer. It steps down system voltage to safe levels such as:

• 110V/√3
• 220V/√3

This voltage then feeds voltmeters and voltage-based protection relays.

That is why CTs and VTs become extremely important inside switchgear systems.

Common Components Inside the Low Voltage Compartment

Many devices work together inside the low voltage compartment. Each component performs a specific function.

Indication Lamps

Indication lamps show the operating status of different equipment. These lamps help operators understand switchgear conditions immediately.

Common indications include:

• Breaker ON
• Breaker OFF
• Breaker TRIP
• Spring charged
• Trip circuit healthy

The indication comes from auxiliary switch contacts connected to the breaker mechanism. When breaker position changes, auxiliary contacts also change their state. That change activates the indication lamp.

Different projects may include many more indications depending on the requirement.

Annunciators

Annunciators indicate abnormal conditions rather than normal status conditions. This becomes the biggest difference between annunciators and indication lamps.

Indication lamps mainly show equipment status. Annunciators mainly indicate faults and abnormal conditions.

Examples include:

• Earth fault
• Short circuit
• Trip circuit unhealthy
• Protection operation

When a fault occurs, the corresponding annunciator window starts blinking. Some systems also include hooters that create audible alarms.

The number of annunciator windows depends entirely on project requirements.

Meters

Meters help monitor electrical parameters continuously. Some panels use dedicated ammeters and voltmeters. Other systems use multifunction meters.

Multifunction meters can monitor:

• Voltage
• Current
• Power factor
• Active power
• Reactive power

Some switchgear panels also include revenue meters for energy monitoring.

Selector Switches

Selector switches help operators choose different operating modes. One common example is the local and remote selector switch.

When the switch remains in remote mode, operators control the breaker from SCADA systems. Local operation becomes blocked.

When switched to local mode, remote commands become disabled. This arrangement also acts as an interlock and improves operational safety.

Another common selector switch is the TNC switch:

• Trip
• Neutral
• Close

Operators use this switch for breaker operation.

Protection Relays

Protection relays become one of the most important devices inside the low voltage compartment. Modern relays provide multiple protection functions within one unit.

These functions may include:

• Overcurrent protection
• Earth fault protection
• Thermal overload protection
• Phase unbalance protection
• Breaker failure protection
• Trip circuit supervision

Some switchgear panels contain multiple relays depending on protection requirements.

Voltage Detection and Indication System (VDIS)

VDIS becomes very important in medium voltage switchgear and ring main units. This system helps operators check whether voltage is present inside the cable compartment.

Suppose maintenance staff need to work on a cable section. Before touching any conductor, they must first confirm that no voltage is present.

VDIS provides that safety indication. Once operators confirm voltage absence, maintenance can begin safely.

This becomes an extremely important safety feature in medium voltage systems.

Understanding an Actual Low Voltage Compartment

A practical low voltage compartment contains many interconnected devices. The upper section generally includes:

• Breaker status indications
• Spring charge indication
• Fault indications

The center section may contain multifunction meters and protection relays. Some relays are withdrawable types and can be replaced easily if faults occur.

Annunciators may indicate:

• Overcurrent trip
• Earth fault trip
• Spare fault windows

The compartment may also contain:

• Electronic hooters
• Local and remote selector switches
• TNC switches
• Reset push buttons

The reset push button acts as a master reset for annunciations and fault indications.

Once engineers open the low voltage compartment internally, they usually see a large number of wires and logic circuits. As more protection functions and interlocks are added, wiring complexity also increases.

That complexity also increases panel cost because additional:

• Wires
• MCBs
• Contactors
• Relays
• Interlocking circuits

become necessary.

Why the Low Voltage Compartment is So Important

Maintenance engineers interact with the low voltage compartment regularly. They normally do not access cable compartments or breaker compartments while the system remains energized.

However, operators frequently access the low voltage side for:

• Checking indications
• Monitoring relays
• Performing local operation
• Reviewing alarms
• Resetting faults

That is why this section becomes extremely important in switchgear systems.

Without a properly designed low voltage compartment, safe operation and protection become impossible.

Conclusion

The low voltage section acts as the operational brain of medium voltage switchgear. It manages protection, monitoring, indication, and breaker control functions safely and efficiently.

Although the main switchgear may operate at 36kV, the low voltage section works safely at much lower voltages using CTs, VTs, and auxiliary supplies.

Understanding this compartment helps engineers troubleshoot problems faster and operate switchgear more safely.

For a clearer and more practical understanding, it is recommended to watch the full video explanation.