IDMT Relay Setting Calculator: Overcurrent Settings (IEC & IEEE)

Free IDMT Relay Setting Calculator. Calculate Plug Setting Multiplier (PSM), Time Multiplier Setting (TMS), and fault operating time for IEC and IEEE curves.

Overcurrent & IDMT Relay Setting Calculator

In industrial power systems, ensuring that a circuit breaker trips at the exact right moment is critical for preventing catastrophic equipment failure and arc flashes. Inverse Definite Minimum Time (IDMT) overcurrent relays are the backbone of this protection scheme.

Unlike an instantaneous trip, an IDMT relay operates on a simple principle: the higher the fault current, the faster the operating time.

Use the calculator above to instantly determine the Plug Setting Multiplier (PSM) and the expected trip time based on global IEC 60255 and IEEE standard characteristic curves.

Understanding the Key Relay Parameters

To accurately set an overcurrent relay (ANSI Code 51), protection engineers must calculate two primary settings:

1. Plug Setting Multiplier (PSM)

The PSM is a ratio that tells you how severe the fault is compared to the relay’s pickup setting. If the PSM is less than or equal to 1, the relay sees the current as normal load and will not trip.

To calculate it manually:

2. Time Multiplier Setting (TMS / TDS)

While the PSM determines if the relay will trip, the TMS (or Time Dial Setting in IEEE) determines when it will trip. It acts as a vertical shift for the tripping curve. A TMS of 0.1 will trip 10 times faster than a TMS of 1.0 for the exact same fault current. This is crucial for protection coordination between downstream and upstream breakers.

How to Calculate IDMT Relay Operating Time

If you need to verify the calculator’s output for a protection study or university lab, the formulas depend on the specific characteristic curve chosen.

For IEC 60255 Standard Curves:

The universal formula for IEC curves is:

Where constants k and Alpha define the curve type:

• Standard Inverse (NI):

• Very Inverse (VI):

• Extremely Inverse (EI):

For IEEE Standard Curves:

North American IEEE curves add a constant (c) to the equation to account for electromechanical reset times:

• Moderately Inverse:

• Very Inverse:

Selecting the Right Curve for Your Plant

• Standard Inverse: Best for utility distribution networks where the fault level doesn’t change drastically based on distance.
• Very Inverse: Ideal for industrial applications where the fault current decreases significantly as the fault moves further away from the main substation.
• Extremely Inverse: Often used for equipment that requires fast protection against overheating, such as large power transformers and heavy-duty cables, as the steep curve closely matches their thermal damage limits.

The High Stakes of Overcurrent Protection

Let’s be honest: protection coordination is one of the most stressful parts of industrial electrical engineering. If you set a breaker to trip too slowly, a localized short circuit can turn into a full-blown arc flash, melting switchgear and putting lives at risk. But if you set it too fast? You end up with “nuisance tripping,” shutting down a multi-million dollar production line just because a large motor started up.

That delicate balance is exactly why we built this IDMT Relay Setting Calculator.

Inverse Definite Minimum Time (IDMT) relays solve the nuisance-trip problem by following a simple rule: The more severe the fault, the faster the breaker trips. A minor overload might take 10 seconds to trip, giving the system time to recover or clear itself. But a massive bolted fault? The relay clears it in fractions of a second.

Also need to decode the Current and other parameter you can use our Electrical Load Calculator

Decoding the Jargon: PSM and TMS

If you are staring at a SEL, Siemens, or ABB digital relay manual right now, you are probably buried in acronyms. Here is the plant-floor translation of the two numbers our IDMT relay setting calculator just generated for you:

1. Plug Setting Multiplier (PSM): “How Bad is the Fault?”

Think of the PSM as the severity multiplier. It compares the actual fault current flowing through your cables to the absolute minimum current required to wake the relay up (the pickup current).

• If your PSM is less than 1, the relay is asleep. It just sees normal, healthy load current.
• If your PSM is 2, the fault is twice the pickup value.
• If your PSM is 20, you have a massive short circuit, and the relay needs to act violently fast.

2. Time Multiplier Setting (TMS): “How Long Do We Wait?”

Also known as the Time Dial Setting (TDS) in IEEE standards, the TMS is your coordination dial. It shifts the entire tripping curve up or down.

Why do we need this? Imagine a fault happens on a minor conveyor belt motor. You want the small breaker right next to that motor to trip first. You do not want the massive main breaker at your primary substation to trip and kill power to the entire factory. By setting a higher TMS on your upstream main breaker, you force it to “wait” and give the downstream breaker a chance to clear the fault first.