Electrical equipment is designed to handle different types of current, and there are mainly three types of current ratings that apply to it: Normal Current Rating, Short Circuit Current Rating (for a specific time period), and Dynamic Current Rating. Let’s explain these in simple terms:
Normal Current Rating
The normal current rating of electrical equipment refers to the maximum continuous current that the equipment can safely carry under standard operating conditions without exceeding its design limits, such as temperature rise, insulation capacity, or mechanical stress. This rating is typically expressed with amperes (A) in RMS and depends on factors such as the equipment’s construction, materials, and cooling mechanisms.
The normal current rating ensures that the equipment operates efficiently and reliably over its lifespan without deterioration or risk of failure. Examples of electrical equipment with normal current ratings include circuit breakers, transformers, and switchgear.
Think of it as the “everyday workload” of the equipment. For example, a transformer or circuit breaker designed for a normal current rating of 1250 A can carry that much current without overheating or wearing out prematurely. This current is of rated frequency (50Hz) and for this current the temperature of the equipment does not cross its specified limit.
This rating is determined as per relevant standards (e.g., IS 13118 for circuit breakers) to ensure compatibility with electrical systems and to maintain safety and performance under normal conditions.
The standard normal current ratings for electrical equipment at different voltage levels are as follows:
- 36 kV Equipment:
- Standard normal current ratings: 630 A, 800 A, 1250 A, 1600 A, 2000 A, and 2500 A.
- 145 kV Equipment:
- Standard normal current ratings: 1250 A, 1600 A, 2000 A, 2500 A, and 3150 A.
- 245 kV Equipment:
- Standard normal current ratings: 1250 A, 1600 A, 2000 A, 2500 A, 3150 A, and 4000 A.
- 420 kV Equipment:
- Standard normal current ratings: 2000 A, 2500 A, 3150 A, 4000 A, and 5000 A.
Short Circuit Current Rating (for a specific time period)
This is the maximum current the equipment can safely handle during a short circuit fault, like when there is an unintended connection of live part of an equipment with the other phases or earth body causing a surge of current. This fault current is much higher than the normal current and lasts for only a short period, usually 1 to 3 seconds. The equipment must withstand this extreme current without being damaged due to mainly overheating until the fault is cleared by protection devices (like circuit breakers). For example, an equipment rated for a 31.5 kA short circuit current for 3 seconds means it can handle a surge of 31,500 amps for up to 3 seconds. This current rating also expressed in RMS only.
The short circuit current for 33 kV equipment is generally 25 kA for 3 seconds, but for some Indian utilities, it is considered 31.5 kA for 3 seconds. For 132 kV and 220 kV equipment, the short circuit current is typically 31.5 kA for 3 seconds and 40 kA for 3 seconds, respectively. For 400 kV equipment, it is 50 kA for 1 second.
| Voltage Level | Short Circuit Current | Duration |
|---|---|---|
| 33 kV | 25 kA or 31.5 kA | 3 seconds |
| 132 kV | 31.5 kA | 3 seconds |
| 220 kV | 40 kA | 3 seconds |
| 400 kV | 50 kA | 1 second |
Short Circuit Breaking Current of a CB
For any equipment, including circuit breakers, the current rating represents the maximum current the equipment must withstand during a fault condition. However, in the case of circuit breakers, they not only need to withstand this fault current but also interrupt it to stop the fault. Therefore, circuit breakers have an additional short circuit current rating, known as the short circuit breaking capacity. This breaking capacity indicates the maximum fault current the circuit breaker can safely interrupt, which matches the short circuit current rating of the breaker.
Dynamic Current Rating
This refers to the equipment’s ability to withstand the instantaneous mechanical forces caused by the high current during a fault (short circuit). When a surge of current flows, it creates strong magnetic forces that can physically stress the equipment. The dynamic current rating ensures the equipment can handle these forces without structural damage. It’s about the equipment’s toughness in handling the immediate “shock” of a fault current. This current is expressed with its peak value. This current rating is calculated as 2.5 times of short circuit withstand current of the same equipment. Accordingly, the dynamic current ratings of various equipment of different voltage levels as follows.
| Rated System Voltage (kV) | Short Circuit Current (kA) | Rated Dynamic Current (kAp) | Value of Short Circuit Current × 2.5 |
|---|---|---|---|
| 33 | 31.5 | 78.75 | 78.75 |
| 132 | 31.5 | 78.75 | 78.75 |
| 220 | 40 | 100 | 100 |
| 400 | 50 | 125 | 125 |
Short Circuit Making Current of a Circuit Breaker
The short-circuit making current of a circuit breaker is the highest current the breaker can handle safely when it closes during a fault, without getting damaged. This current rating is usually the same as the breaker’s dynamic current rating.