The 4M400 Test Block is an evolution of our popular 4M300 system. The main difference is that each of the fourteen (14) test groups may be specified at time of order to provide automatic CT shorting and sequential circuit to suit specific protection schemes:
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 4M400 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 4M400 and allows a single model 4M420 Test Plug to be employed for all 4M400 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.
Changing the 4M400 Test Block from the normal service condition occurs in three (3) stages as depicted in the timing diagram figure 1.
Where more than 14 test circuits are required such as in EHV transmission protection panels, the 4M402 Test Block with 28 test circuits may be employed.