Types of Overcurrent Relays and Their Application

Overcurrent Monitoring Relay

A current monitoring relay is used for the type of relay that will protect against excessive current, either caused by an electrical fault or some other type of overcurrent condition.

But faults can be either high current or low current. And while many current monitoring relays can detect both, some will only sense one type of current or the other.

An overcurrent monitoring relay is placed in line with the circuit it is to protect. If the current in the circuit being monitored rises above the threshold set on the overcurrent relay, then the relay will “trip” and open the circuit. This prevents damage to equipment or injuries to people.

Types of Overcurrent Relays

There are several different types of overcurrent relays available on today’s market. These will mostly differ in their features and functions, but some also differ in terms of how they are designed and what type of overcurrent they are meant to protect against. The overcurrent relay types are:

1. Instantaneous Overcurrent Relay

The first type of overcurrent relay is the instantaneous overcurrent relay. This type of relay is designed to protect against very high levels of current for a very short period of time (less than 0.1 seconds). These types of currents can be caused by faults in the system, such as a short circuit.

The instantaneous overcurrent relay will have a very low time delay, meaning that it will trip or open the circuit as soon as the current is detected. This is important, as it can help to prevent damage to the electrical system by quickly removing the source of the overcurrent.

Applications for these types of overcurrent relays include outgoing feeders, busbars, and transformers. In these applications, the instantaneous overcurrent relay can provide protection against faults that could otherwise cause extensive damage.

2. Inverse Time Overcurrent Relay

The second type of overcurrent relay is the inverse time overcurrent relay. Unlike the instantaneous overcurrent relay, the operating time is, as the name suggests, inversely proportional to the current magnitude.

That means a high current will reduce the amount of time it takes for the relay to operate, while a lower current will result in a longer operating time, as long as 10 seconds. This is important, as it can help to prevent false trips when there is only a momentary surge of current.

When it comes to application, these types of overcurrent relays are mostly used in distribution lines. Inverse time overcurrent relays are further classified as inverse definite time relay, very inverse relay, and extreme inverse relay based on their time delay characteristics.

Normal Inverse Time Overcurrent Relay

These types of overcurrent relays offer a lower or smaller change in the operating time as the current magnitude changes. They are, therefore, often used industrial situations and utility systems.

Very Inverse Time Overcurrent Relay

A very inverse time overcurrent relay will have a much greater change in the operating time as the current magnitude changes than the normal inverse time overcurrent relay. They are, therefore, well-suited for use in applications where the fault current is expected to be lower.

Extremely Inverse Time Overcurrent Relay

An extremely inverse time overcurrent relay has the greatest change in operating time as the current magnitude changes of all the inverse time overcurrent relays. They are, therefore, much faster to respond to fault situations. These types of overcurrent relays are commonly used to prevent protecting cables, motors, transformers, and other equipment such as heaters and pumps from overheating.

Long Time Inverse Overcurrent Relay

Based on its name, you can easily tell that the long time inverse overcurrent relay has an extremely long operating time. In fact, it can take up to 60 seconds for this type of relay to operate.

That being said, the long time relay is not as sensitive as other types of overcurrent relays and is, therefore, mostly used in installations where there is a low risk of faults. These overcurrent relay types are mostly installed to protect against earth faults.

3. Definite Time Delay Overcurrent Relay

3. Definite Time Delay Overcurrent Relay

This type of relay is similar to the inverse time delay overcurrent relay, in that it only trips or opens the circuit when the current is above a certain level for a specific amount of time. However, the definite time delay overcurrent relay has a fixed or set time delay.

This means that the amount of time it takes for the relay to trip or open the circuit is always the same, regardless of the level of current. This can usually be set by the user. These overcurrent relay types operate independently of the amount of current, provided it’s above the predetermined value.

Definite time overcurrent relay characteristics allow it to be used in applications such as transmission lines and transformers where the tripping must happen after a specific time delay.

4. Directional Overcurrent Relay

Directional overcurrent relays are commonly used to protect against faults in a system where the current may flow in either direction. For example, they may be installed to respond to faults in a ring main system.

Directional overcurrent relays are able to detect the direction of the current flow and will only operate when the current is flowing in a certain direction. This helps prevent unnecessary trips or circuit breaker operations in systems with several current paths or power sources.

Based on its operation, the directional overcurrent relay is used for protection of circuits in these systems:

• Ring Main Unit (RMU)
• Radial Distribution System
• Generator Protection

The directional overcurrent relay can be used in conjunction with other relays to provide complete protection for the system.

Conclusion

Overcurrent monitoring relays are essential devices when it comes to the protection of electrical systems. They can help to prevent damage to equipment and components by quickly removing the source of the overcurrent. There are several different types of overcurrent relay. Each of these has its own unique benefits and best applications. Depending on your type of application, you’ll need to select the right type of overcurrent relay to ensure adequate protection.