Standard Operating Procedure (SOP) for LT Panel

What is a Low Tension (LT) Panel?

The LT Panel is an electrical distribution board typically accepts power from a generator (or) transformer and distributes it to different electronic devices & distribution boards.

What is the use of LT Panel?

LT Panels employ low tension cables to draw power from the generator (or) transformer & distribute it across different electrical devices & distribution boards. 

LT panels are meant to operate at lower voltages (up to 440 volts) with low insulation levels.

Objective 

To establish procedures for operating and maintaining low tension (LT) panels. 

Scope 

This SOP is applicable to all low-tension panels. 

Responsibilities

Operator/Engineer/ Executive

Equipment Details

The LT-Panel comprises of the following equipment:

1. Metal enclosed free standing section
2. Vacuum circuit breaker
3. Bus bar
4. Control wiring

Construction of LT Panel

• Modular, 
• Metal-enclosed, 
• Free-standing compartmentalized panels 

are appropriate for indoor and outdoor installations. Panel boards are made from thick mild steel structural sections. 

The frames must be surrounded by 2.0 mm cold rolled steel. 

Cables enter each chamber from the top or bottom via replaceable gland plates. Suitable gland plate-terminal support is supplied in the cable chamber. 

All covers, doors, and joints have neoprene gaskets.

Air Circuit Breakers of LT Panel (Designed for circuits under 600 V)

Circuit breakers prevent over-currents from 

• Short circuits (or) 
• Ground faults in (or) near equipment. 

Power distribution systems and associated components need protection from 

• Overcurrent, 
• Overvoltage, 
• Undervoltage, 
• Reverse current flow, & 
• Unbalanced phase voltages. 

Circuit breakers are installed in electrical distribution systems. If the circuit breaker detects an overcurrent condition, its contacts will open, preventing current flow to the machine. 

Air circuit breakers are prevalent in electrical distribution. An air circuit breaker connects an electrical power supply to a load. 

Component is called the main contact assembly. A main contact can be opened to interrupt power flow from the source to load, (or) closed to allow power flow. 

A low voltage air circuit breaker’s movable contact is positioned on a contact arm that is pivoted to open contacts by a spring-powered operating mechanism activated by a trip unit in response to an overcurrent condition in the protected circuit. 

Many air circuit breakers control compression springs with mechanical linkages among a latching shaft & an actuation device. 

This device can be manually (or) electrically controlled. An air circuit breaker can be mounted between power supply and load or drawer type, which can be moved to enable maintenance and ensure stability.

Busbars of LT Panel

All busbars are air-insulated and composed of high-conductivity Tin-coated copper or Electrolytic Aluminum. 

The fault resist capability of the busbar is 25 KA/35 KA/50 KA 1 Sec, which corresponds to the rating of the incoming circuit breaker or as otherwise specified. 

All bus bars are fully shielded by PVC sleeves in a distinct compartment along the length of the panel and have expansion holes. 

Suitable separation shall be established between busbar chamber & adjacent compartments. 

TP and N busbars will be used, with the neutral bus section being full size or half the phase busbar size as needed. PVC sleeves with red, yellow, blue, and black stripes will cover the busbar. 

The busbar shall be isolated and braced with insulated supports (DMC / F x RP / S x MQ) at appropriate intervals to withstand short circuit electromagnetic stresses. 

According to 15 and IEC standards, phase, neutral, and body electrical clearances must be maintained.

Control Wiring

Panel will have PVC-insulated 1.1 KV grade, multi-strand flexible copper conductor (1.55sq.mm) cross section internal wiring. 

A phase’s wiring will be red, yellow, (or) blue, earthing will be green, and neutral will be black. Insulated straps with minimum compression will cleanly lay and run wiring. 

All control wiring for equipment operation, protection relays, and meters has crimped terminations and glossy plastic ferrules at both ends.

LT Panel Operation

Auto Operation

• Check all three phases incoming supply.
• For transformer power, keep the LT Panel Auto/Manual switch in auto position.
• Check the charged ACB Spring.
• Transformer ACB closes automatically.
• Under voltage/short circuit/EB power failure trips the transformer ACB.
• DG set starts, closes DG breaker, and charges LT panel automatically.
• Once EB power returns, DG sets ACB trips and EB ACB closes automatically.

Manual Operation

• Check the 3-phase power supply.
• Maintain LT panel Auto Manual switch in the “Manual” position.
• Make sure that EB supply ACB spring is charged. (If not. Charge manually).
• The “Close” push button on the LT panel closes the EB supply ACB if it is not closed.
• Check all protection relays.
• Check for the cause of the relay trip and reset it after fixing it.
• If that is not closed, hit the ACB “Close” push button to close it.
• Ensure the DG-ACB spring is charged. (If not manually charged).
• Transformer ACB trips if EB power fails/under-voltages/shorts.
• Manually start & close DG set ACB.
• Trip the DG, set ACS manually, & “CLOSE” EB ACB after power returns.

Standard Operating Procedure for LT Panel

• Turn off the power supply in the area where you plan to work. 
• Provide a “Men at Work Do Not Switch On” board. 
• If necessary, remove the fuses with a fuse’s puller. 
• Be sure you’re standing on an insulated rubber mat. 
• Wear proper footwear when performing the work. 
• Bring the necessary instruments, such as an insulated plier, tester, wire remover, test lamp, and fuse puller. 
• Before handling any conductor, make sure it is dead. 
• Execute the maintenance. 
• Before charging the supply, be sure that no tools of any kind are left inside the panel. 
• Close the panels’ doors. 
• Exit the workplace only after completing an acceptable trial. 
• The low-tension panel requires preventive maintenance every six months.

Procedure for closing the Air Circuit Breaker (ACB)

• Check that the breaker is in the right location inside the panel. 
• Confirm that the “TRIP CIRCUIT HEALTHY” indicator shows on the panel. 
• Using the provided volt meter, check the incoming voltage across R-Y, Y-B, and B-R; it must range between 415 to 430 volts when applicable. 
• Charge the closed-circuit breaker by moving the handle on the breaker back and forth. 
• Repeat the procedure. 
• until the breaker’s spring indicator shows that it is charged. 
• Ensure that the breaker’s spring indicator shows that the spring is charged. 
• To close the circuit breaker, press the green close button. 
• In the case of Auto mode, the breaker will be constantly charged. 

Procedure for Opening an Air Circuit Breaker (ACB) 

• To open the air circuit, press the red open button. 

Procedure for Closing the Main Feeder Circuit 

• Rotate the feeder handle clockwise from 0 to 1.
• Make sure the red color indication is “ON”. 

Procedure for opening the Main Feeder Circuit 

• Rotate the feeder handle counterclockwise from 1 to 0. 
• Make sure the red color indicator is “OFF”.

Insulation Resistance Test

The insulation resistance test is a direct current evaluation test that provides an accurate indication of presence of moisture, pollution, or breakdown in the insulation. 

This test also provides a baseline for future comparative tests.

It contains a panel enclosure, busbar, CT, and circuit breaker. 

Before starting testing, it is important to take the following measures.

• A visual inspection makes sure that surface dust & moisture are removed/clear from the component being tested.
• Ensure the section is segregated from other parts of the system that may feed into untested sections or circuits.
• During testing, apply voltage to one phase and connect it to ground. Repeat the process for the other phases described above.

The following table shows the test voltage limits:

Contact Resistance Test

Contacts should be checked on a regular basis to prevent resistance from arcing, which can reduce current carrying capacity. This can help determine the quality of the contact surfaces & pivot points. Contact resistance should be kept as low as possible to prevent localized heating & power losses.

• Confirm busbar joints are correctly connected and tightened.
• The test must be performed with CBs inserted & closed.
• Calculate contact DC resistance between panels using 100A DC. This will result in the evolution of busbar joint, CB cluster resistance, CB contact resistance, & CT main resistance (if appropriate).
• Ensure consistent outcomes throughout all phases and measurements.
• Factors to consider include measured track length, busbar rating, CB and CT, and temperature.

High Voltage Test

• Ensure equipment is in good working order and prevent deterioration that could limit its lifespan.
• To conduct the test, remove all connections to earth circuit.
• Electronic equipment for control, signal, and instrumentation, as well as DC control equipment, must be disconnected or removed from the control circuit during testing.

Checklist of SOP for LT Panel 

A	General check	Check Type	Acceptance Conditions	Yes/No
1	Visual Inspection	Physical	Observe
2	Dimensional Check	Measurement	Observe
3	Check the BOM	Verification	Observe
4	Paint Thickness Check	Measurement	Observe
5	Verification of the Type Test Certificates	Verification	Observe
6	Verification of the Brought Items Test	Verification	Observe
7	Busbar Calculation Sheet	Verification	Observe
8	Recommendations for Protecting Relay connections	Verification	Observe
9	Wiring Check (Standards and Connections)	Verification	Observe
B	Routine Test	Check Type	Acceptance Conditions	Yes/No
1	Insulation Resistance Test (Before and After HV tests)	Measurement	Observe
2	HV Test	Measurement	Observe
3	Polarity Tests (CT & PT)	Measurement	Observe
4	Primary Injection and Ratio Tests	Measurement	Observe
5	Breaker Operational Testing (Electrical & Mechanical)	Physical	Observe
6	Functional Test	Physical	Observe
7	Numerical Relay Configuration & Functional Checks	Physical	Observe
8	Sequential Test	Physical	Observe

Advantages of Switchgear Testing

• Reduce company risks by adhering to regulatory regulations and standards.
• Improve the quality & safety of the products & procedures.
• Save time & money.
• Collaborate with a regulatory specialist to reduce time-to-market & improve market access.

Standards

IEC 61439: This standard outlines the design verification process for LT control gear assemblies, as well as the duties of the manufacturer & switchboard maker.