Level 2 — Troubleshooting Bench

Equipment and sources

• Controller: Allen-Bradley ControlLogix 5590, 1756-L902TS (Rockwell 1756-TD001: one USB port and two embedded EtherNet/IP ports; 1.20 A at 5.1 V). Only its standard task is used: the 1756-IB16 and 1756-OB16E are standard I/O, not safety I/O, and the program below is not a safety function.
• Rack: 1756-A4 chassis; 1756-PA72 supply (85–265 V AC, input 100 VA maximum, inrush 20 A maximum, hold-up 6 cycles at 120 V: 1756-TD005J p.3; terminals and fusing instruction: 1756-IN619G p.5); 1756-N2 filler in the empty slot (1756-TD006).
• I/O: 1756-IB16, 16 sinking inputs, ON from 10 V; 1756-OB16E, 16 sourcing outputs, 1 A per point, electronically fused per group of 8 (1756-TD002U pp.45 and 102–104; RTB pin assignments read from its wiring drawings). A group fuse trip turns the whole group off and sets FuseBlown bits that stay latched until reset (1756-UM058L p.36 and p.123). Terminal blocks 1756-TBNH. Backplane load 1.20 + 0.10 + 0.25 = 1.55 A at 5.1 V of the PA72’s 10 A.
• Control power: transformer 1497A-A8-M6-0-N, 350 VA; F5/F6 Bussmann FNQ-R-1-8/10 and F1 FNQ-R-4 are the Table 13 and 14 maximums of 1497-TD001 for 350 VA at 480/120 V. 24 V DC supply 1606-XLP72E (Class 2); F7 Bussmann LP-CC-4.
• Motor circuit: contactor 100-C09EJ10 with an electronic 24 V DC coil (closing and opening delays 20–50 ms: Rockwell 100-TD013, Table 19); overload 193-T1AB40; 2 hp, 460 V motor (nameplate FLA 3.4 A; NEC Table 430.250 also lists 3.4 A for 2 hp at 460 V: the fuses are sized from the table, the overload from the nameplate) on the 480 V system.
• Rack grounding: the chassis protective-earth stud (14 AWG) and its functional earth at the leftmost bottom mounting tab (8 AWG) each run to their own bolt on the ground bus (1756-IN621E pp.11–14); the PA72’s PE terminal goes to the ground bus (“Attach to chassis ground lug or ground bus”, 1756-IN619G p.5).
• Terminal blocks: on a real 1756-TBNH the conductors are dressed down to the strain-relief area at its bottom and tied there (1756-IN062F p.6). The enlarged breakouts draw each conductor leaving sideways so that every screw stays visible and probeable.
• Photoeye: Allen-Bradley 42JT-P2LAT1-A2 VisiSight, polarized retroreflective with reflector 92-125, 2 m cable: 10–30 V DC, 30 mA maximum, output 100 mA maximum, off-state leakage 10 µA maximum (42JT-IN002B p.5), well inside the IB16’s 1.5 mA off-state limit. Its black output sources into the IB16 and, at the factory dark-operate setting, is ON while a box blocks the beam (p.2; the p.1 table labels the same behavior “Light Operate”, a wording conflict in the maker’s document). The white remote-teach lead is not connected. The picture is a generic photoeye.
• Fault light: Allen-Bradley 800FP-P4PN3R, a 22 mm red LED pilot light for 24 V AC/DC, 24 mA at 24 V DC, terminals X1/X2 (800-TD008G pp.5 and 9). The OB16E’s 1 mA off-state leakage (1756-TD002U p.104) is below the light’s 3 mA leakage rating, so an off output does not light it.
• Run feedback: M1_Aux comes from a front-mounted bifurcated auxiliary block, Allen-Bradley 100-FCB11 (contacts 23-24 N.O. and 31-32 N.C., for C09 contactors with the integral 13-14), rated for switching down to 5 V / 3 mA (100-TD013Q pp.31 and 47). The contactor’s integral 13-14 is rated only down to 17 V / 10 mA, so it is left spare. Whether the IB16 input always draws at least that 3 mA is an open item (below).
• F7: Bussmann LP-CC-4, Class CC time-delay, 600 V AC / 150 V DC (Bussmann data sheet 1023).
• Pictures: the rack, modules, terminal blocks and E-Stop are training illustrations, not product photos; every probe ring is placed on the picture’s own screw.

How the controller avoids an unexpected restart (each case is tested on this page)

• The E-Stop’s contact 11-12 and the overload’s 95-96 open M1’s coil string in hardware. The program only reports and permits.
• The start permit (rung 7) is granted only while Start and RUN are both released, then holds itself. Stop (door or HMI), the E-Stop, a trip or a fault drops it. So a Start or RUN held through a stop, an E-Stop release, a RESET or power returning never starts M1: it has to be released and pressed again.
• Pickup window, 150 ms (rung 1): if M1’s aux contact has not confirmed within 150 ms of the run command, the start is dropped and a fault latched. If the output supply is missing at the press, or drops while M1 is still closing, and returns inside the window with Start still held, that same start completes late: the motor never ran, so it is a delayed start, not a restart. Past the window, the start is aborted.
• Established run (rungs 2 and 3): once confirmed, any loss of M1_Aux while the run command is on drops the permit in the same scan, with no timer, and latches a fault.
• Output group fuse (rung 4): any group 0 FuseBlown bit drops the permit (all eight bits are tested, because which bits a group trip sets is not documented).
• Latched M1_Fault (rungs 5 and 6): RESET FAULT clears it only once every cause is gone, and RESET never starts M1.
• Power: the rack supply holds the backplane up for 100 ms; a longer loss stops the controller. At power-up the firmware prescan clears the run command and the permit, so Start must be released and pressed again.

Open items (not claimed)

• Switch-on inrush: Rockwell gives the PA72’s 20 A and the XLP72E’s 26–30 A peaks with no duration or I²t, so whether F1 and F5/F6 ride through switch-on, and T1’s voltage during it, are NOT verified. Closing this needs the makers’ inrush duration or I²t, or a measurement on real hardware.
• The PA72 instructions call for a 15 A time-delay fuse in each ungrounded supply conductor; this branch is protected by F1 (4 A time-delay). The two are not reconciled here, and no compliance claim is made.
• Run-feedback contact current: Rockwell publishes the IB16’s on-state current only as 2 mA minimum at 10 V and 10 mA maximum (3.12 kΩ maximum impedance) at 31.2 V (1756-TD002U p.46). Treating the input as a resistor gives about 7.6 mA at 24 V, but that is an estimate, not a verified minimum for the 100-FCB11’s 3 mA rating; it needs Rockwell’s input curve or a measurement.

Why the overload is found the way it is

• A normal overload trip also opens 95-96 and closes 97-98, so those states alone do not prove a failed relay. The lesson asks for its own RESET after it has cooled (it will not latch), a check that the motor circuit is not the cause (balanced line-to-line winding readings), and the open 95-96 shown with the ohmmeter on the de-energized circuit.
• The controller never holds the coil output on without run feedback, so the coil string cannot be traced live. The ohmmeter is used instead, and its readings count only after zero energy has been verified under your own lock and tag.

Simplifications in this simulation

• The program reads its inputs once per 10 ms scan. A real module delivers its data asynchronously, at its RPI, so production code buffers inputs at the start of the routine. An interruption shorter than about one scan plus the input filter may not be seen.
• Controller start-up is shortened to 4 s. The contactor’s minimum OFF time is not modeled.
• Ohmmeter: readings are solved through every parallel path of the de-energized circuit. The transformer windings use typical values (Rockwell publishes none), and the motor uses a typical 10.9 Ω line to line (a comparable 2 hp 460 V motor’s maker data: 10.857 Ω, Baldor-Reliance EM3558T, typical, not guaranteed). Electronic loads (M1’s coil, the pilot light, the supplies and the I/O circuits) are left out and read OL. Rockwell publishes no coil resistance for the 100-C09EJ10’s electronic coil.
• The bimetallic overload’s cooling time is not modeled: the lesson states that the line stopped about 20 minutes before your RESET attempt.
• No fault in this lesson trips the output group fuse; that path is exercised by the page’s tests.

This page is a training simulation. It is not an electrical-safety certification of any real circuit.