Protective Device Clearing Time Calculator

Protective-device clearing time is the elapsed time between a fault or overcurrent condition and interruption by the protective device. In coordination work, the clearing-time value may come from a time-current curve, a relay study, test data, or another established protection calculation.

The Protective Device Clearing Time Calculator compares a known clearing-time value against a reference limit. It produces two planning values:

• Clearing time margin — the amount of time remaining between the measured clearing time and the entered limit.
• Clearing time ratio — the measured clearing time expressed as a multiple of the entered limit.

The result provides a direct pass/fail-style clearing-time comparison for a stated time criterion. It does not determine the clearing time of a breaker, fuse, relay, or protective-device system.

Clearing-Time Inputs

The calculation uses two time values in seconds.

Input	Electrical meaning
Measured clearing time	The known protective-device clearing time for the protection screen, entered in seconds. This value may be measured or derived from a protective-device curve.
Entered clearing-time limit	The maximum permitted clearing time for the same protection screen, entered in seconds.

Both values must represent the same electrical condition. For example, a curve-derived clearing time at a specified available fault current should be compared only with a limit applicable to that same condition.

A clearing-time comparison can support broader protection review where a maximum clearing time has already been established. It may be used as a documented check alongside equipment protection requirements, selective coordination studies, arc-flash engineering inputs, or project-specific protection criteria. The calculator itself does not establish those criteria.

Calculation Method

The calculator subtracts the known time from the entered limit:

\(\displaystyle \text{Clearing time margin} = \text{Entered clearing-time limit} - \text{Measured clearing time}\)

It also calculates the proportion of the allowed time that the measured value consumes:

\(\displaystyle \text{Clearing time ratio} = \frac{\text{Measured clearing time}} {\text{Entered clearing-time limit}}\)

A positive margin and a ratio at or below 1.0 indicate that the entered clearing time is within the entered limit. A zero margin indicates that the clearing time is exactly at the limit. A negative margin or ratio above 1.0 indicates that the entered clearing time exceeds the entered limit.

Calculation Example

For a protection screen with the following entered values:

Field	Value
Measured clearing time	0.08 s
Entered clearing-time limit	0.10 s

The time margin is:

\(\displaystyle 0.10\text{ s} - 0.08\text{ s} = 0.02\text{ s}\)

The clearing-time ratio is:

\(\displaystyle \frac{0.08\text{ s}}{0.10\text{ s}} = 0.8\text{ x}\)

Result: The measured clearing time is 0.02 seconds below the entered limit and uses 80% of the allowed clearing time. The clearing-time comparison is: Entered clearing time is within the entered limit.

Protection-Study Application

The calculator is useful after a time value has already been established. Common source values include:

• A total-clearing-time reading obtained from a protective-device time-current curve.
• A relay or breaker protection-study output at a defined fault-current point.
• A test result recorded for a protection setting or equipment review.
• A project worksheet value evaluated against an owner, engineer, equipment, or study-defined time limit.

Clearing time can affect several downstream engineering decisions. A shorter interruption time may be relevant to conductor short-circuit withstand review, equipment damage exposure, incident-energy calculations, and upstream/downstream coordination analysis. Those evaluations require their own source data and methods; a time margin alone does not validate them.

For conductor work, ampacity, AWG or kcmil selection, insulation temperature rating, terminal rating, adjustment factor, correction factor, current-carrying conductors, voltage drop, and raceway fill are separate design checks. Protective-device clearing time may be an input to a fault-duty or thermal-withstand review, but it does not replace branch-circuit or feeder conductor sizing.

Field and Code Limits

This worksheet performs curve arithmetic only: it compares an entered measured or curve-derived time with an entered limit.

It does not perform protective-device coordination, verify breaker or fuse selection, calculate available fault current, establish relay settings, evaluate arc-flash hazard, determine conductor withstand, or approve an installation for code compliance. Device curves, actual settings, fault-current assumptions, equipment ratings, manufacturer documentation, project specifications, and AHJ requirements must be reviewed separately.

Use clearing-time values from the correct protection condition and confirm whether the reported time represents the intended quantity, such as total clearing time rather than a partial operating interval. Final device and installation decisions require a qualified electrical design and field review.