How does Trip Circuit Supervision enhance Electrical Safety?

What is a Switchgear?

A switchgear is a device (or) combination of devices that is primarily designed for the purpose of generating, carrying, and stopping currents in electric circuits, both under normal conditions and during abnormal situations. Switchgears can be classified as either single-purpose devices or as combinations of single-purpose devices. Therefore, in the case of that a variation, such as a defect, occurs in the network, the identification and immediate removal of the malfunctioning part of the network is an extremely essential function that any switchgear can carry out.

Trip Circuit in Switchgear

The trip circuit in a switchgear system is of vital significance.

Even if an installation includes  protection relays, auxiliary relays, as well as  switchgear, if the circuit breaker trip coil is broken &/or the wiring is loose &/or the power supply to the circuit breaker trip circuit is interrupted,

In that condition that the trip circuit acquires a defect, the fault will either not be isolated, which will result in catastrophic damages, or it will lead to the tripping of an ahead switching device, which would result in an unplanned interruption in power in a larger portion of the network. Both of these results are undesirable.

What is Trip Circuit Supervision?

The

• Primary protection relay,
• Master trip relay,
• Relay contacts,
• Switchgear auxiliary contacts,
• Fuses, linkages,
• Wiring,
• Terminal blocks,

and so on can all be found in the trip circuit of a switchgear. Given the large number of components, there is a significant risk of the trip circuit acquiring a fault at some point. As a result, there is a requirement for a system that can monitor the trip circuit’s integrity. The short form for this is “TCS,” which stands for “trip circuit supervision.”

Working Principle

Function-1

In its most basic form, trip circuit supervision can consist of a ‘Trip Circuit Healthy (TCH)’ indication lamp, as seen in the image below.

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 In the case of a short circuit in the light, a series resistor (R) is connected to the circuit to prevent the breaker from tripping.

This configuration provides trip circuit supervision only if the breaker is turned on.

When the circuit breaker is closed, the trip circuit supervision scheme is activated.

Funtion-2

The trip circuit supervision (TCS) function can easily be stretched to the breaker OFF position by implementing a basic integration, as demonstrated below:

When the circuit breaker is open (or) closed, the TCS Scheme is in effect.

Function-3

The trip circuit supervision (TCS) is continuous in both of the above conditions. Because this is basically a local lamp indication of trip circuit health, an operator must constantly monitor the glowing of the TCH lamp. This is not a feasible option. Furthermore, with resistors in the circuit, such continual supervision of the trip circuit may result in an inappropriate demand on the sub-station battery capacity.

As a result, the circuit can be adjusted so that the trip circuit condition is only tested ‘on demand.’ The installation of a ‘Trip Circuit Healthy Check (TCH)’ push button with a normally open (N.O.) contact arrangement enables this and which is illustrated below:

Adapted circuit with the push button denoted as TCH.

Ex: However, the above scheme’s shortcoming is that it would only provide a local signal. As such, it is only appropriate for switchgear installations that are maintained. Additionally, the operator must periodically verify that the trip circuit is in good working order by pressing the “TCH Push Button” and making sure the TCH lamp illuminates. Even if the trip circuit is malfunctioning, the concerned personnel won’t be made aware of the concerning condition if the operator has neglected their duties in any way.

It resulted in the catastrophic failure of vital equipment in such arrangement (like the ones mentioned above) featuring a “TCH check push button & a TCS Lamp.” Trip coil failures of significant breakers in the installation had occurred.

Funtion-4

A Trip Circuit Supervision (TCS) Relay is a device that can be utilized to both prevent the occurrence of such a fault and provide a remote alarm in case the trip circuit’s integrity is disrupted.

In the event that the circuit breaker is either open or closed, the trip circuit supervision (TCS) scheme (with remote alert) will activate.

Function-5

• When the breaker is turned on and the trip circuit is functioning properly, the current path will be as follows: ‘+ ve’, Fuse, ‘R’, ‘coil A of the TCS relay, ’52a’ contact, Trip Coil (TC), Link, and ‘-‘ve.
• coil A of the TCS relay will be activated, & the coil normally open (N.O.) contact, which is wired in the path of TCS relay coil C, will close. As a result, coil C of the TCS relay will be activated, opening its normally closed (N.C.) contact, which could be linked to the trip circuit failure alarm.
• When the breaker is turned off and the trip circuit is in good condition for operation, the current path is as follows: ‘+ ve’, Fuse, ‘R’, ‘coil A of the TCS relay, ‘R’, coil B of the TCS relay, ‘R’, coil C of the TCS relay, ‘R’, coil C of the TCS relay Trip Coil (TC), Link, and ‘-‘ve are all possible configurations.
• Both coil A and coil B of the TCS Relay will now be activated, and these two coils normally open (N.O.) contacts which are wired along the path of coil C of the TCS Relay, will close. As a result, coil C of the TCS relay will be activated, opening its normally closed (N.C.) contact, which could be connected to the trip circuit failure alarm.
• As a result, while the trip circuit is functioning properly, no trip circuit failure warning is triggered. An alert will be triggered if the trip circuit is unhealthy, regardless of whether the breaker is in the ON or OFF position.

Again, resistors are included in series with coils A and B to prevent unwanted tripping in the occurrence of an unintended short circuit in one of the trip circuit supervisory circuit components. These resistors are often installed independently, exterior to the relay. It is important to remember that the trip circuit failure alarm supply must be independent of the tripping supply to ensure that the trip circuit failure indication/alarm is obtained even if the tripping supply fails.

Thus, the Trip Circuit Supervision (TCS) relays are designed to provide continuous supervision of the circuit breaker trip circuit and to provide an alarm in the case of a loss of auxiliary supply, a fault on the trip-coil (or) it’s wires independent with respect to the breaker position, a fault on the breaker auxiliary contacts, or a fault in the supervision relay itself.