The potential of about -5V on the KT lead generates KP and KP bar

(but not KF and KFbar) in unit 2.  KP prevents premature generation of the 'path switched' function PS (PS-generation required that the P wire and hence the KT lead is earthed}, and KP operates the 'subscriber parked' bistable SP/sP. KP also gives warning of permanent application of potential

to a KT lead by operating relay KTW and hence lighting the MARK WELD lamp if

KP is present in the absence of the release-guard function RG.

In unit 3 SP, is conjunction with SC generates SPS ( 1start primary

selection) and this, via current amplifying transistors VT16, VT18 and

VT19, sends an interrogating signal via the Primary COS .Amplifiers, the

COS Strapping Field, and Route Relays I to the Primary-route Supervisory

Relay Sets. At the end of the chain of Supervisories the signal returns

through Route Relays I and continues as a Selector-starting signal on

lead SST to the Band C Switch Selectors.

Function SPS in unit 3 also earths Register lead MK to prepare

for connection of the Register to the Supervisory about to be chosen.

This is a fault condition, probably caused by a 'tappint 2 fault

on the line, ie a loop followed almost immediately by a disconnection. The

KF,/KF bistable in unit 2 operates, generating KFR and, in unit 3, KFR. In

unit 1 KFR, in conjunction with the 'start of call' function SC, generates

TS and TSR to record the fault on the TAPPING SUBSCRIBER meter and, by

earthing Register lead PBY, release the call to plant-busy tone.