HT PANEL

HT Panel; Components of HT panel; Working of HT panel; Features of HT panel;

Full form of HT Panel –

The full form of HT panel is High Tension panel.

What is HT panel –

Definition of HT panel is given below-

Definition – HT panel is a metal enclosure fitted with HT Circuit Breakers, relays & metering that is used to receive 11KV/33KV supply (from one or more source) & distribute the power through its outgoing feeders. Outgoing feeder may be one or more it depends on the load of the building.

Full form of HT panel is High Tension Panel. It is installed in Electrical substations of Commercial complexes, residential colonies, factories, schools, hospitals etc. to receive & distribute HT supply.

Main functions of HT panel –

To make & break HT supply (or switch on or switch off supply),
To receive & distribute HT supply,
To provide protection against faults such as short circuit, overload, earth fault, overheating of transformers winding, overheating of transformer oil.
To provide metering to monitor various parameters.

Main components of HT Panel –

HT panel consists of following components –

Circuit breakers – these circuit breakers are used for switching ON / OFF HT supply (11KV or 33KV). The switching On/OFF may be done manually, automatic through relays, locally or remotely. Different – different circuit breakers are available such as Vacuum Circuit Breaker (VCB), SF6 (Sulphur Hexa Fluoride) Circuit Breaker & Oil type circuit breakers, but VCB is mostly used in HT panels. A HT panel may have one or more circuit breakers depending on the requirement of electrical distribution system. The circuit breaker may be operated mechanically (through a push button) and electrically (through a switch).
Relays – Different relays may be used in a HT panel to protect electrical system from fault such as earth fault, overlading & short circuits. Different relays are used to protect from different faults. The function of relay is to sense the fault & give tripping command to circuit breaker of the panel. WTI & OTI relays are used to protect transformers from over heating of winding & Oil.
Meters – Different meters may be installed on HT panel to measure/record various parameters such as incoming voltage, current, power factor, energy consumption, Load in Kw etc. These parameters may be measured through multifunction meter or individual meters.
CT & PT – Current transformers (CT) & Potential transformers (PT) are also important component of HT panel. The function of CT & PT is to reduce the actual value of current & voltage respectively to a very low value so that low values could be used for metering & protection purpose.

For example, 200A ampere current can be reduced to 5A or 1A. Similarly, 11KV voltage can be reduced to 110V.

A typical HT panel components detail is given in below diagram –

Above diagram shows various indication lights, relays, meters, switches which are used in HT panel. (Note – Types of relays, switches, meters, number of indicating lights etc depend on the type of HT panel feeder e.g. incomer feeder, bus coupler & outgoing feeder.)

Let’s understand the function of each component installed in HT panel –

R,Y,B Indicating light – These indicating lights are used to see incoming supply (HT) in the panel. All three lights glow when incoming supply is healthy (all three phases are available). All these lights will go off when incoming supply is interrupted from source end. If any one indicating light is off then that particular phase is not available & need investigation for the same.
Breaker On/Off indication light – These are used to know the breaker’s On/Off status. Red indication is used for breaker’s ON status & green for OFF status. When breaker is off, green indicator will glow & red will not glow. When breaker is switched on, green indicating light gets off & red light will be ON.
Trip indicator – This is used to know the breaker’s trip status. This light remains off till breaker remains healthy & ON. When breaker trips on fault, trip indicator light will glow.
Spring Charged indicator – This is used to know whether spring of breaker is charged or not for next operation. Spring is an integral part of VCB operation. Let’s us understand like this, suppose VCB is off & operator wants to switch it on so before switching of VCB, charging of spring is must. Charging of spring can be done manually (with the help of handle) or electrically (with the help of motor which is installed in the breaker itself). Once spring is charged (indicator will be ON), breaker can be switched on either by pressing the button (manually) or by TNC switch (electrically). The moment VCB is switched ON, Spring gets immediately discharged (indicating light will be off). Therefore, it is important that spring should be charged again for next operation of VCB. The charging of spring is done automatically (immediately after switching on VCB) with the help of spring motor.
Trip circuit healthy indicator – This indicator is required to know the health of the trip status. If trip circuit is faulty/not healthy, breaker will not be tripped on fault. Therefore, indicator is On when trip circuit is healthy , it means trip circuit is healthy otherwise indicator will be off. (Note- The meaning of trip is the switching off the breaker automatically on fault.)
DC fail indicator – This indicator shows the status of DC supply which is required for control wiring of HT panel. Control wiring is done for different purpose such as for tripping, relay control etc. Normally 24V Dc is used in HT panel for metering & protection purpose. If DC circuit is not OK then required DC supply will not reach to meters & relays & these items will not work.
OC & EF Relay – This relay may be electronic type or electromagnetic type. The purpose of this relay to trip the VCB on over current (which may be over loading or short circuit) or on earth fault. These relays are set on a particular value to trip breakers if fault current (OC or EF) crosses the set limit.
Master Trip Relay – This relay is installed in HT panel which is integrated along with all other protective relays. This relay is manual reset type. This relay has a flag which drops on fault & this down flag doesn’t allow VCB to switch on. Therefore, the purpose of this relay is to make sure that operator has checked the fault & rectified it before switching on the breaker. This relay can be reset by just lifting the flag after opening the front cover of the relay. This relay avoids to switch on power if fault persists in the system.
Energy Meter – This meter records & display total energy consumption of the building/facility load.
Multifunction Meter – This meter displays many live parameters of electrical supply such as Voltage, KW, KVA, KVAh, power factor (pf), frequency, line current (in R-phase, Y-phase, B-phase) etc.
Annunciator Panel – The function of this panel is to show the type of fault at one location. Each annunciator panel has multiple windows & each window is assigned for different fault. The annunciator panel is associated with one hooter which is installed at the top of the HT panel. The purpose of hooter is to alert the operator who is away from the HT panel. Also it is connected with three push buttons named Test, Accept & Rest. When a fault occurs in the system, this fault is displayed by one window of annunciator panel & hooter starts sounding.
Test, Accept & Reset buttons – These buttons are part of annunciator panel. Test button – it is used when healthiness of annunciator panel is checked. When we press test button then annunciator panel’s all window will glow & hooter will sound. If it’s not happening it means some fault is there in the system which needs to be checked. Accept button – It is used when fault occurs, electrical supply is disconnected when breaker trips on fault & hooter starts sounding on fault & one window is showing fault. When operator press accept button, hooter stops sounding but fault window will continuously glow. The operator then starts investigating the fault. Reset button – it is used when system has been normalized after clearing the fault. When this button is pressed, the fault window stops glowing & electrical supply can be switched on again for normal operation.
Voltage selector switch – This is used to check incoming supply in terms of voltage such as voltage between R & Y phase, Y & B phases, B & R – phases by changing the knob. Three stages are there to see different voltages between phases.
T-N-C switch – This switch is used to operate VCB. It is an electrically operated switch which has three stages T-N-C. T means Trip (switch off) the breaker manually by rotating knob anti-clockwise. N is neutral position (vertical) where knob remains vertical. C means close the breaker by rotating knob clockwise.
Emergency switch – This is push & lock button which is used for tripping/switching off breaker manually in case of emergency by pressing it & leave the button in pressed position by locking it so that no other person can operate the breaker.

Working of HT panel –

HT panel in electrical system is a switching point where HT supply can be connected or disconnected with load. HT cables are connected to circuit breakers from incoming side & outgoing side also. HT supply can be switched on or off by operating electrically operated switch (or by using push button of the breaker). When supply is ON, metering system starts measuring/recording electrical parameters (depending on the panel design).

HT panel provides protection also from various faults (depending on the fault protection relays installed in the panel). When fault occurs in the system HT panel is switched off automatically (this process is also known as tripping). Each relay gives fault indication when it operates. Before switching on HT supply again, fault should be identified & rectified. If fault persists in the system, then relay will not be reset & HT panel can’t be switched on because of interlocking between relay & panel. Therefore, it is very important to clear the fault first & then switch on HT supply again. (Note- Relay should not be bypassed for any reason).

Features of HT panel (for 11KV or 33KV) –

HT panel receives electrical Supply 11KV or 33KV) from H – pole by 3 core HT cable & then it distributes power through one or more outgoing feeders.
These outgoing feeders are connected with distribution transformers which convert HT supply into 415V, 3-phase 4 wire AC supply.
To measure electrical power consumption, Energy Meter is installed in it,
Meters are installed to measure electrical Voltage, Current, frequency, power factor etc.
Transformer protection relays are installed in it such as –

For Dry type transformer – WTI (Winding Temperature Indicator) – Alarm & Trip,

For Oil type Transformers – WTI & OTI (Oil Temperature Indicator) – Alarm & Trip and Buchholz relay (gas operated relay) – Alarm & Trip.

Short circuit protection, over load protection, earth fault protection relays can be installed in it against protection from faults.
HT Circuit Breakers such as VCB, SF6, etc are available in market but VCB is the most commonly used HT breaker are used to make & break HT supply due to less maintenance.
HT circuit breakers can be switched ON & OFF either manually or electrically.
24V (or 30V) DC supply (battery charger) which is an external source to HT panel is used for various purposes like for metering, relay operations, indicating lights,
240 V ac supply is also used in HT panel for 16A power sockets provided inside panel, for panel lights & space heater.
Space heater is provided in HT panel to avoid moisture inside the panel.
To measure Incoming Supply voltage, 11KV is converted into 110V through PT (Potential Transformer). And 110V voltmeter is used for voltage measuring purpose.
To measure building load in terms of current, total current of the building is converted into 5A or 1A through CT (Current Transformer). CTs are available in different range – 600/5A, 400/5A, 300/5A ….. or 600/1A, 400/1A, 300/1A …… A typical picture of HT panel is given below –

Safety requirement in HT panel room –

Single line diagram (SLD) – One SLD must be installed in HT room which must contain incoming & outgoing feeder details of the building.
Rescue hook – one rescue hook should be available at suitable location in the HT panel room so that in case of emergency/shock, it could be used. The purpose of rescue hook is to separate a person from electrical supply if he comes in contact with it.
Discharge rod – on discharge rod should be available in the HT panel room. The purpose of discharge rod is to discharge the HT cable ends & HT cable terminals when cable is disconnected with HT panel due to any fault or maintenance purpose.
Don’t touch uninsulated parts of HT panel after switching of HT supply of panel – uninsulated parts of HT panel should not be touched before discharging them. Because HT busbars & cables may store charge during continuous operation. Stored energy may be harmful & give shock.
Hand Gloves – Suitable voltage rating (11Kv 33KV) of hand gloves should be available in HT panel.
Shock treatment chart– It should be displayed in the HT room which shows that how a person is given first aid treatment when he gets an electric shock.
Panel Keys – Panel keys should be available at suitable location in HT panel room (very close the HT panel).
Door Lock – Panel door should always be closed & locked condition. This helps to prevent foreign objects / insects inside HT panel. Entry of insects in HT panel may lead to short circuit in the panel.
Holes/openings – All holes/openings should be sealed properly to prevent foreign objects / insects inside HT panel. Entry of insects in HT panel may lead to short circuit in the panel.
Identification of feeders – all incoming & out going feeders of HT panel should be numbered from front & back side. Both sides (front & back) of one feeder should have same number. This numbering & leveling prevents accidents during maintenance.
Danger plate – Suitable rating of danger plate (11KV or 33KV) should be installed on HT panel.
Lock HT room – HT panel room should be locked in case no manpower is available in it & key should be kept at suitable location.
Entry of authorized person – A list of authorized person should be displayed at the entrance of HT room. Only authorized should be allowed in HT room.
Maintenance – A regular maintenance should be carried out of HT panel as per schedule by OEM or trained inhouse manpower.
Testing of relays – Relays installed in the HT panel should be checked on regular basis to ensure that breaker will trip in case of fault.

Note – NBC 16 (National Building Code of India 16) provides complete details of electrical system which are required to follow when a building is constructed. Different sections cover different requirement of electrical system.

Electrical HT Panels related guidelines are given in the link below –

NBC 2016- Planning of electrical installations

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