Corporate | Railway | Industry

4M402 Flexible and high performance test block system


  • Finger safe test sockets suit standard or shrouded type 4mm banana plugs for direct access to the protection or measurement scheme
  • 28 independent test groups may be specified with any combination of circuit types
  • Automatic shorting of CT circuits completed in the test block – No test links or operator intervention required
  • Isolation plug provides sequential circuit isolation timing in three (3) stages
  • Clear and concise front panel circuit identification
  • Test plug fitted with insertion handle and thumb screw retention system to enhance operator safety and system security
  • Side label instructions on test plug for changing from normal service to the test condition
  • High current / voltage rating
  • Made in Australia


The 4M402 Test Block is an evolution of our popular 4M300 system. The main difference is that each of the twenty eight (28) test groups may be specified at time of order to provide automatic CT shorting and sequential circuit isolation to suit specific protection schemes:

  • STAGE I Test Block Cover Extraction
    Isolation of Type 1 circuits
  • STAGE II Isolation Plug Extraction
    CT Shorting Automatic shorting of all CT circuits
    Isolation of Type 3 circuits
    Isolation of Type 2, 8 and 9 circuits
  • STAGE III Test Plug Insertion
    4mm Test Sockets available

The main advantage of this approach is the improved level of safety and security afforded to the CT circuits. This is because the CT shorting function takes place within the 4M402 Test Block irrespective of the CT circuit position. In many test block systems the CT shorting is only accomplished when the Test Plug is inserted which leaves open the possibility of a CT circuit becoming open circuit due to the CT shorting links being omitted or in the wrong position. This potential problem is negated in the 4M402 and allows a single model 4M422 Test Plug to be employed for all 4M402 Test Block configurations.

Each test circuit is connected to a separate pair of terminals at the rear of the case. During normal operation of the associated protection equipment, each terminal pair is connected via a circuit-shorting link.

4M402 Flexible and high performance test block system


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Buffalo Grove, IL 60089-6918
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Graycar Business Park
Barton under Needwood
Burton on Trent
DE13 8EN
United Kingdom
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