SSSI – Smart SSI Speed Limit Switch

Sapcon’s SSSI – Smart SSI speed limit switch is a versatile switch suitable for detecting an over-speed or under-speed condition of rotating, reciprocating or conveying equipment. It can also detect a missing object amongst a series of continuously moving objects at a constant speed, such as a missing bucket in a bucket elevator.

Working of SSSI – Smart SSI Speed Limit Switch

The sensor works on the principle of reluctance. Whenever the motion detection sensor
detects a ferromagnetic target, a signal in the form of a pulse is given to the microprocessor (in the evaluation unit). The microprocessor evaluates the time between consecutive pulses and converts it into ppm (objects per minute). When the speed becomes less than the programmed ‘under-speed­’ value, the under-speed­ relay changes the state. When the system rotates at a constant speed, the time gap between two consecutive pulses remains constant. Now, if an object (and hence pulse) is missed, this time gap increases. This is sensed by the microprocessor and it operates the missing object relay. The relay output contacts can in turn be used for signaling or controlling. Flashing LED in the preamplifier indicates that the moving target is within the sensing range and facilitates target­-to­-detector distance setting.

Follow us on LinkedIn


The speed monitoring system comprises of a motion detector probe (MDP) with a Pre-amplifier (PA) and an evaluation unit (EU). When MDP is suitably installed to sense the moving target, the PA outputs pulses, the frequency of which is proportional to the speed of the moving target. These pulses are superimposed on the power supply feeding the PA and are transmitted to the EU. The EU after processing the pulses, compares their duration with stable internal time reference. This duration is sensed, amplified and made to operate a relay whose contacts in turn can be used for annunciation / control.

This has a specially designed probe suitable for temperatures up to 120°C (standard probe) and up to 200 °C (special probe), with a large sensing range of 100mm working on the reluctance principle. It has SS casing with an aluminium mounting flange and a tri-clamp arrangement for easy adjustment of target distance. An electronic insert is housed in the cast aluminium probe head provided with a suitable cable gland. An electromagnetic shield covers the rear of the probe to minimize interference from stray electromagnetic fields.

  • Easy to install and simple to calibrate
  • Over-speed / Under-speed alarm, field selectable, with fail-safe relay operation
  • Flashing LED in the pre-amplifier indicates the moving target is within the sensing range and facilitates target to detector distance setting
  • Normal, Alarm and Power Failure, indicated by LED’s
  • Accurate pulse interval evaluation by comparison with a stable internal time base
  • Built-in adjustable start up delay with interlocking provision allows the monitored device to accelerate before the speed is evaluated
  • Flashing LED in the evaluation unit confirms receipt of pulses
  • Rugged motion detector probe especially designed for heavy industrial application
Mechanical Specifications



  • Cast Aluminium, Weatherproof, Stoving enamel painted, suitable for back / wall mounting

Cable Entries      

  • 3 Nos. of ½” BSP / NPT

Operating Temperature

  • 20°C to 60°C



  • Plastic with built-in terminals.


  • 20°C to 80°C 12 V at 17 mA max. derived from evaluation unit only

Operating Temperature

  • 20°C to 80°C


Probe Head         

  • Cast Aluminium weather-proof


  • Cast Aluminium Flange

Target Range Adjustment

  • By tri-clamp mechanism

Maximum Detection range

  • 100 mm

Target Material

  • Ferromagnetic

Minimum Target Area

  • 20 x 20 cm

Temperature Range

  • 120°C (Standard)
Electrical Specifications



  • 110 / 230 V AC (-10 to +10%) 50Hz (as specified in the order)

Power Consumption            

  • 5 VA approx

Relay Output            

  • One set of potential free change over contacts rated at 6A, 230V AC 50Hz for non-inductive load.

Power to Pre-amplifier        

  • 12V DC at 50 m A max

Alarm Mode

  • Overspeed / Underspeed field selectable

Measuring Range     

  • 2 to 3000 Pulses / Min. (PPM)

Set Point Adjustment           

  • Coarse setting by jumpers, fine setting by potentiometer

Start-Up Time Delay            

  • 0 to 60 Second Adjustable by means of a potentiometer


  • By potential free NO contact of contactor controlling the Motor of the monitored equipment

Overall Dimensions  

  • 180 x 180 x 85 mm




  • 12 V DC at 50 mA max. derived from Evaluation Unit


  • Current pulses
Applications Specifications

Detects speed of conveyors and Bucket Elevators and signals an alarm if the speed falls below the set speed. Special sensor detects moving ferrous objects from 100 mm distance. It can also be used as a missing bucket detector. High temperature models upto 200°C are available.

Switching Indication

Red / Green LED’s for Alarm and Normal conditions respectively