Enter the relay pickup (trip) current and the feeder load current into the calculator to determine the relay pickup setting as a percentage of the feeder load current.
Over Current Relay Setting Formula
The following equation is used to express an overcurrent relay pickup (trip) current as a percentage of the feeder load current (i.e., as a “multiple of load”). Note: real-world relay settings also depend on CT ratio (if used), curve selection, time dial/TMS, and coordination with other protective devices.
RS = \frac{OC}{FLC}\times 100- Where RS is the relay pickup setting (as % of feeder load current)
- OC is the relay pickup (trip) current (amps)
- FLC is the feeder load current (amps)
To calculate this pickup setting percentage, divide the relay pickup (trip) current by the feeder load current, then multiply by 100.
What is an Over Current Relay Setting?
Definition:
An overcurrent relay “setting” is not just one number; it commonly includes at least a pickup current (or pickup as a percent/multiple of a CT-rated current) and a time setting/curve for coordination. This page’s calculator specifically computes the pickup current expressed as a percentage of the feeder load current.
How to Calculate Over Current Relay Setting?
Example Problem:
The following example outlines the steps and information needed to calculate the relay pickup setting as a percentage of feeder load current.
First, determine the relay pickup (trip) current. In this example, the pickup current is set to 200 amps.
Next, determine the feeder load current. For this problem, the feeder load current is 100 amps.
Finally, calculate the over current relay setting using the formula above:
RS = (OC / FLC) × 100
RS = (200 / 100) × 100
RS = 200%
FAQ
What is the purpose of an overcurrent relay in an electrical system?
An overcurrent relay is a protective device used in electrical systems to detect excessive current and initiate tripping of a circuit breaker (or another disconnecting device), thereby helping protect equipment and conductors from damage due to overloads or short circuits. It operates by measuring current and issuing a trip signal when current exceeds a predetermined level for the selected time characteristic.
How does the setting of an overcurrent relay affect system protection?
The setting of an overcurrent relay determines the level of current (and, for time-overcurrent elements, the time curve/setting) at which the relay will operate to trip the circuit. A properly set relay supports good protection and coordination by responding to true overcurrent conditions while avoiding unnecessary trips during normal load or acceptable inrush/temporary overloads. Incorrect settings can either lead to frequent false trips or fail to protect the system during actual overcurrent events.
Can overcurrent relays protect against both overload and short-circuit conditions?
Yes, overcurrent relays can protect against both overload and short-circuit conditions when their elements are applied appropriately. Overload protection is typically provided with an inverse-time characteristic set slightly above normal operating current. Short-circuit protection is often provided by a high-set/instantaneous (or very fast) element, where provided, to respond quickly to severe fault currents.
