Voltage and frequency protection relay
The frequency and voltage protection RFT-451A is dedicated to be used in protection system of generators and frequency load shedding or voltage drop automation systems in order to prevent against system unstable conditions. Digital design of relay provides high accuracy, stability and dependability.

Manuals, diagrams, technical details


Product description

Protection set

81                           over-/underfrequency protection

81S                        rate of change of frequency by frequency derivative (df/dt)

81, 81S                 frequency and rate of change of frequency (df/dt) with settable operation logic AND/OR

81S, 81SA            frequency and rate of change of frequency (by average rate of change of frequency ∆f/∆t) with settable operation logic AND/OR

27                           time-delayed undervoltage, three phase

59                           time-delayed overvoltage, three phase


Main features

  • Three phase voltage measuring circuit
  • 5 settable stages of frequency protection, each with possibility to set one of four operation modes
  • 3 settable stages of voltage protection, each with possibility to set one of two operation modes
  • Function of auto-reclose after under-frequency-load-shedding operation
  • Voltage restrain of frequency protections
  • Visual indication of key device statuses
  • Current values of voltage and frequency measurment
  • 3 binary inputs to provide external protection signals or to block operation of any of function
  • 7 programmable relay outputs
  • Real time clock
  • Event recorder of 62 unique events with capacity of 300 events
  • LCD display with buttons set in order to operation of the relay
  • Watchdog contact to signal improper operation of the relay or lack of auxilliary voltage
  • RS-485 communication port for remote control with MODBUS protocol (optional)
  • Self-test of proper operation of relay
  • Relay enclosure gives possibility to mount on 35 mm rail, surface mount or flush mount

The RFT-451A device is digital frequency and voltage protection relay. Energizing quantities are: RMS value of voltage, voltage fundamental frequency, frequency rate of change (derivative) and average change in time. If any of measured values exceeds setting value, the relay operates in a manner depending on configuration.

With optional communication interface, the measured currents, binary inputs or outputs, register logs are provided. The PC software provided with relay allows to set the relay, confirm generated signals, graphical display of measured values and browse the register records.


Rated voltage Un 100V or 230V
Rated frequency fn 50Hz
Rated auxilliary voltage Up 24V DC, 48V – 60V DC,
110V – 230V DC/AC
Setting range of frequency protections:
f (40 ÷ 65)Hz
df/dt (-10 ÷ +10)Hz/s
f AND/OR df/dt (40 ÷ 65)Hz and (0,1 ÷ 10)Hz/s
f AND/OR Δf/Δt (40 ÷ 65)Hz and (0,2 ÷ 10)Hz/s
Setting range of threshold voltage (5 ÷ 200)V by step of 0,1V if Un=100V
(10 ÷ 460)V by step of 0,1 V if Un=230V
Setting range of restrain voltage of frequency protections Ubl Ubl (0,1 ÷ 0,8)Un by step of 0,1Un
Setting range of timer
(0,00 ÷ 99,99)s by step of 0,01s for frequency and voltage protection
(0,02 ÷ 2)s for ∆f/∆t protection
Accuracy of frequency measurement 0,01Hz
Accuracy of df/dt measurement 0,1Hz/s
Accuracy of ∆f measurement 0,02Hz
Accuracy of voltage measurment 0,5% in the range of (0,8÷2,0)Un
1% in the range of (0,2÷0,8)Un
2,5% in the range of (0,05÷0,2)Un
Accuracy of timers 1%
Burden in measurement inputs ≤1VA/input
Burden in supply module ≤6W
Pick-up time of underfrequency protections ≤100ms
Pick-up time of overfrequency protections ≤120ms
Pick-up time of df/dt protection (0,10 ÷ 0,64)Hz/s <500ms
(0,65 ÷ 1,62)Hz/s <260ms
(1,63 ÷ 4,02)Hz/s <200ms
(4,03 ÷ 10,0)Hz/s <180ms
Pick-up time of voltage protections ≤60ms
Reset time of frequency protections ≤100ms
Reset time of voltage protections ≤100ms
Continuous voltage withstand of voltage inputs 1,2Un
Relay outputs data:
Continuous contact carry 6A
Maximum breaking capacity – AC1 1500VA/250V
Maximum breaking capacity – DC1 28V/220V 6/0,16A
Maximum breaking capacity  ind. load (L/R ≤40ms) 0,1A/250V DC
Ambient temperature (268÷313)K (-5÷40)°C
Relative humidity ≤80%
Ingress Protection degree IP40
Weight ~ 0,8kg
Dimensions (height x width x depth) 75 x 100 x 120 mm
Electromagnetic compatibility according to EN 50263
Insulation according to EN 60255-5

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