Understand the practical differences between IDMT vs DMT Relay, and Instantaneous overcurrent relays. Learn their characteristics, applications, and protection use cases. with this post IDMT vs DMT Relay will be going to very easy for you

IDMT vs. DMT Relay Characteristics: Practical Applications & Use Cases

Walk into any modern industrial substation, open the switchgear panel, and you will see a digital numerical relay staring back at you. While these devices look like simple computers, they are the last line of defense between a minor electrical fault and a catastrophic switchgear fire.

The core job of an overcurrent relay (ANSI 50/51) is to sense a fault and tell the circuit breaker to trip. But the critical question protection engineers face is: How long should we wait before tripping?

To answer that, we have to choose the right protection characteristic. Let’s break down the practical differences between Definite Minimum Time (DMT), Inverse Definite Minimum Time (IDMT), and other critical relay parameters used on the plant floor.

Definite Minimum Time (DMT) Relays

The Rule: I don’t care how massive the fault is; I am going to wait exactly “X” seconds before I trip.

With a DMT relay characteristic, the operating time is constant and entirely independent of the fault current magnitude. Once the fault current crosses the pickup threshold, an internal timer starts. Whether the fault is 1,000 Amps or 10,000 Amps, if the timer is set to 0.5 seconds, the relay will trip in exactly 0.5 seconds. Here is why we need to know IDMT vs DMT relay behavior.

Practical DMT Applications and Use Cases

• Transmission Lines: In long transmission networks, the fault current doesn’t change drastically whether the fault is 10 km away or 50 km away. DMT relays are excellent here because you can use “time grading.” You set the breaker furthest down the line to trip in 0.2s, the next one upstream at 0.5s, and the main feeder at 0.8s. This guarantees the closest breaker isolates the fault.
• Backup Protection: DMT is frequently used as a solid backup protection scheme for large transformers. If the primary differential protection fails, the DMT relay will definitively clear the fault after a set time delay, regardless of the fault size.

Inverse Definite Minimum Time (IDMT) Relays

The Rule: The bigger the fault, the faster I trip.

This is the workhorse of industrial electrical systems. An IDMT relay has an inverse relationship between the fault current and the operating time. A minor overload might take 10 seconds to clear, but a massive bolted short circuit will be cleared in 0.1 seconds.

Practical IDMT Applications and Use Cases

• Motor Protection: Large 3-phase induction motors draw massive starting currents (up to 6x normal load) for a few seconds. If you used an instantaneous relay, the motor would never start. An IDMT relay’s inverse curve allows it to “wait out” the starting current spike, but still provides rapid protection if a true short circuit occurs.
• Distribution Networks: In standard industrial plants, fault currents drop significantly the further you get from the main transformer. Because IDMT relays trip faster for higher currents, they naturally coordinate themselves. The downstream breaker (seeing a smaller fault current due to cable impedance) will naturally trip slower, giving the upstream breaker a chance to hold steady.

(Note: If you need to calculate exact trip times for IEC or IEEE IDMT curves, use our free [Link to your IDMT Calculator Page] tool).

Decoding IDMT Curve Types: Standard, Very, and Extremely Inverse

If you look at the settings on a Siemens or ABB numerical relay, you won’t just see one generic “IDMT” option. You will usually have to choose between three primary IEC curves: Standard Inverse (NI), Very Inverse (VI), and Extremely Inverse (EI).

To a junior engineer, these just look like different lines on a graph. But on the plant floor, picking the wrong curve means the difference between a minor localized trip and a massive facility blackout.

Here is how to choose the right curve for your specific equipment.

1. Normal Inverse (NI) / Standard Inverse

• The Shape: The most gradual slope of the three. A significant increase in fault current only results in a moderate decrease in trip time.
• The Plant Floor Translation: This curve is forgiving. It doesn’t overreact to moderate fault spikes, but it also isn’t the fastest at clearing massive short circuits.
• Best Use Case: Utility feeders and long distribution lines. On a long utility line, the fault current doesn’t change drastically whether the fault is 1 km away or 5 km away. Because the fault levels are relatively flat, you use Standard Inverse curves to rely heavily on strict “Time Grading” (TMS) to ensure coordination between substations.

2. Very Inverse (VI)

• The Shape: Steeper than the Standard Inverse. The trip time drops rapidly as the fault current increases.
• The Plant Floor Translation: This is the industrial workhorse. In a factory network, cable runs are relatively short, meaning a fault at the main switchgear will pull a massive amount of current, but a fault at the end of the line (due to cable impedance) will pull much less.
• Best Use Case: General industrial distribution networks. The Very Inverse curve takes advantage of this changing fault current. It naturally grades itself: it will trip lightning-fast for a severe fault close to the main breaker, but will purposely slow down for a minor fault at the end of the factory, allowing the downstream breaker to clear it first.

3. Extremely Inverse (EI)

• The Shape: The steepest curve available. It takes a long time to trip for minor overloads but reacts violently fast to heavy short circuits.
• The Plant Floor Translation: This curve is specifically designed to mimic the “Thermal Damage Curve” (the melting point) of heavy electrical equipment.
• Best Use Case:Motor protection, transformers, and expensive cables. * For Motors: Large induction motors pull huge starting currents. The Extremely Inverse curve is slow enough at the lower end to let the motor start without a nuisance trip, but steep enough at the high end to instantly kill the power if the motor actually shorts out.
  • For Transformers/Cables: When heavily overloaded, cables and transformers heat up exponentially (I^2R losses). The EI curve trips just before the copper reaches its melting point, saving you hundreds of thousands of dollars in replacement costs.

The “Golden Rule” of Curve Coordination

When you are setting up protection coordination across your facility, always try to use the same curve shape for breakers in series.

If your downstream motor breaker uses an Extremely Inverse curve, but your upstream main breaker uses a Standard Inverse curve, their lines will cross on a Time-Current graph. This “curve crossing” guarantees a nuisance trip—meaning your main facility breaker will trip before the motor breaker does, shutting down your entire plant.

IDMT vs DMT Relay: The Quick Comparison

Feature	Definite Time (DMT)	Inverse Time (IDMT)
Operating Time	Constant (Independent of fault size)	Variable (Inversely proportional to fault size)
Fault Clearing	Slower for heavy faults near the source.	Extremely fast for heavy faults near the source.
Best Used For	Transmission lines, constant fault levels.	Distribution lines, motor starting, varying fault levels.
Coordination Method	Strict Time Grading.	Current & Time Grading.

Other Critical Protection Relay Parameters

While IDMT vs DMT Relay dictate the timing, numerical relays have other vital functions you will use in your protection scheme:

1. Instantaneous Overcurrent (ANSI 50)

There is no intentional time delay here. If the current spikes above a massive threshold (usually set way above normal load and motor starting currents), the relay sends a trip signal immediately (typically in under 50 milliseconds).

• Use Case: Protecting cables and busbars from catastrophic, high-energy short circuits where waiting even a fraction of a second could cause an arc flash explosion.

2. Directional Overcurrent (ANSI 67)

Sometimes, protecting against how much current is flowing isn’t enough; you need to know which way it is flowing. Directional relays look at the phase angle between voltage and current.

• Use Case: Ring main systems or facilities with their own solar/generator power. If the main grid fails, you don’t want your facility’s internal generators feeding power backward into the utility grid. A directional relay senses the reverse power flow and trips the tie breaker.

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