Specifications

December 2008 Page 17–3
Chapter 17. Pilot Wire Interface (HCB/SPD) Module
17
Figure 17–3. HCB/HCB-1 Interface Module
for Three-Terminal Applications.
Figures 17-3 and 17-4 show the rear interface
modules for three-terminal line applications.
These modules are double width because, for a
three-terminal line application, you must attach
one three-terminal interface to two adjacent HCB
modules. This double-width rear interface module
properly connects the two HCB modules together.
As with the two-terminal interface modules, the
SPD version has a 4/1 matching transformer
(enclosed in a bright metal can) mounted on it,
while the HCB/HCB-1 version does not.
Figure 17–4. SPD Interface Module
for Three-Terminal Applications.
Another important difference between
HCB/HCB-1 and SPD applications is in the
connection itself. When you connect the module
to an existing
HCB/HCB-1 application, you will
see a ground on one of the pilot wire leads
somewhere between the relay and the old insu-
lating transformer. This ground must be removed.
The appropriate ground connection is made inside
the interface module attached to the rear of the
FOCUS chassis.