The Test Block type 4M300 comprises fourteen (14) test circuits, each of which is connected to a separate pair of terminals at the rear of the case. During the normal operation of the associated protection equipment, each pair of terminals are connected together by a circuitshorting link. Changing the 4M300 Test Block from the normal service condition to the test condition is described below & depicted in figure 3 in the technical bulletin.

Test Circuit Access
Access to the circuits, for testing purposes, is gained by first removing the front cover. For the 4M300-B model the Isolation Plug is withdrawn & the circuit between terminals 13 & 14 interrupted. By routing the main DC supply to the protection scheme or relay through this circuit, removal of the Isolation Plug will thereby prevent inadvertent tripping of the protection during the ensuing tests.

Test Plug Insertion
Insertion of the Test Plug type 4M320, isolates the live side circuits from the equipment side. The Test Plug carries 28 4mm ‘finger safe’ test sockets. These sockets are suitable for shrouded or standard 4mm banana plugs. Each test socket is identified by a number, which corresponds to the numbered terminal on the rear of the case when the Test Plug is inserted. The test socks are colour coded - BLACK to indentify the equipment side sockets & YELLOW to identify the live side sockets.