PCC-IV Instruction Manual (rev. 0.2)
Page 2-20
Section 2 - Installation
DC Commons and AC Neutrals Isolation
The drawing on the next page shows the relationships between the various grounds and commons within the PCC-
IV. A thorough understanding of these relationships will prevent unintended ground loops in the DC and AC field
wiring.
“Ground loops” is a general term to describe unintended current flows via AC ground or DC common circuits,
which generally are not drawn on a schematic as a distinct field wiring connection.
DC ground loops can cause lower than expected 4-20 mA values due to a portion of the current bypassing the
intended circuit path and traveling through some unintended parallel path. AC ground loops can cause Ground Fault
Interrupter Circuit Breakers to trip if current returns via a neutral that is not protected by the GFCI breaker.
Green Wire Ground: The incoming AC power ground
(the “Green” wire) is only connected to case ground (i.e., the
metal enclosure). Case ground is isolated from all neutrals and all DC commons. Case ground should always be
connected to the appropriate power supply ground, per NEC.
Line: The CPU AC power line input (Terminal 2U) is internally connected to CPU terminals 2L and 3L. CPU Terminal
2U is not internally connected to any of the option boards. If a connection is desired, external wiring is required.
Hot: The hot power i
nputs to the “G” and “S” Triac Output option boards are isolated from the CPU line input. Each
option board hot is isolated from the hot of the other option boards. If a connection is desired, external wiring is
required. The hot input for a Triac option board is a common input that feeds all Triacs on that board.
Neutral: The CPU AC Power neutral input (Terminal 1U) is internally connected to CPU terminal 1L. CPU Terminal
1U is not internally connected to any of the option boards. If a connection is desired, run a field wire from the CPU
board to the option board.
CPU Terminal 1U is the return path (or neutral) for all current that enters the CPU 120 Vac discrete input channels
(Terminals 4L-8L). The hots and neutrals
of all “J” option board 120 Vac discrete input channels are isolated from
the CPU board line and neutral. Eac
h “J” board input channel is isolated from other “J” board channels. The “J”
board is provided with jumpers to allow connecting the neutrals of the channels together if desired.
DC Common: The DC com
mons (“-”) are isolated from case ground. All signal commons on the CPU board are
interconnected and are connected to the signal co
mmons on the “A”, “F”, and “G” option boards. Specifically, the
following terminals are all internally connected to each other:
CPU:
9, 12, 14, 18, 20, 22, 24, 26
“A”:
x-41, x-43, x-45, x-47, x-49, x-51
“F”:
x-41, x-43, x-45, x-46, x-47, x-49, x-51
“G”:
x-51
Power Common (PC): Terminal 9 on the CPU b
oard is reserved for “noisy” electrical loads such as external relay
coils, and solenoids.
Signal Common (SC): Except for Terminal 9 (power common), all other DC commons are referred to as signal
common and should be connected to low noise 0-5 Vdc and 4-20 mA circuits.
Isolated Signal Commons:
The “-” terminal of each channel of a “B” option board is isolated from all other channels
on that option board and from the CPU b
oard. The “+” terminals are also isolated channel-to-channel and from the
CPU board.
Isolated RS485:
Terminal 3 (“COM”) is the isolated DC common of the isolated power supply that powers the field
wiring portion of the isolated RS485 communications link. The RS485 d
ata “+” and “-” (terminals 1 & 2) are also
isolated from the CPU board.
The PCC-IV is not powered by 120Vac. Terminals 2U, 2L, 3L are connected to each other but not to any
other internal PCC-IV circuitry. These terminals are provided for compatibility with PCC-III controllers. 120
Vac does NOT need to be connected to these terminals for new PCC-IV installations.
120Vac Neutral DOES need to be connected to 1U or 1L for 120 Vac DIN’s 1 thru 5 to operate correctly
(terminals 4L
– 8L).
The following information is provided to assist installers with a PCC-III to PCC-IV retrofit only.
