PROJECT DESIGN REVIEW — MINUTES OF MEETING
Project: automation island with nine machines, one PLC cabinet
Review subject: cable tray routing, separation, bend radius and future spare capacity
Date: 4 August, 09:00 to 10:40, meeting room 1
Document ref: DMR-2026-0804-CAB09
Attendees
| Name | Role |
|---|---|
| C. Rossi | Lead electrical / machine designer (chair) |
| B. Akande | Panel and cable assembly lead |
| F. Delgado | Machine builder, island integrator |
| H. Nakamura | Control systems engineer |
| M. Silva | Safety / EMC engineer |
1. Purpose of the review
The island groups nine machines around a common PLC cabinet, and the cable count had drifted from the tender figure of 140 cables to a drawing list of 212. The draft tray layout was designed around the 140 count, with one main tray and two sub-trays, and the review was called after the panel shop reported that cables were being routed through gaps in the enclosure because the trays were full before the fifth machine was wired. The review subject was not the cable list alone but the whole management system: tray sizing, power and signal separation, bend radius at the machine entries, and the spare capacity rule that keeps the island serviceable.
2. Review comment CB-01: the fill ratio, which the draft missed
B. Akande opened with the number that stops a tray being a cable landfill. A cable tray should be sized so the total cross-section of the cables occupies no more than forty percent of the tray area for new installs, and fifty percent is the hard limit where adding one more cable means pulling half the bundle. The draft tray was a 300 by 100 mm ladder tray specified at forty percent fill for 140 cables of average 9 mm diameter. The 212-cable reality, with the extra servo, safety and network cables, pushed the same tray to 61 percent fill. F. Delgado noted the symptom: the fifth machine’s servo cables were routed around the outside of the tray because the inside had no lane left, and those outside cables sat against the sharp edge of the tray flange where the flexing killed two of them in the first month.
3. Review comment CB-02: power, signal and safety separation inside the same tray
M. Silva brought the EMC rule before the sizing debate. Servo power cables carry switched PWM current and radiate into the adjacent signal cables if they share a lane without a divider, and the safety circuits have their own requirement, that they be physically separable by a route that cannot be bridged by a single dropped tool. The draft laid one 300 mm tray with no internal divider. The review split the routing into three streams with a fixed divider stripe in the tray: power and servo drives on one side, encoder and network signals on the other, and the safety pair in a separate closed duct 40 x 60 mm run above the main tray. H. Nakamura agreed, with the note that the island had already seen a ground-loop alarm from a shared power-signal lane on the earlier station, and that the cost of the divider inside the tray is a stamping, not an engineering decision.
4. Review comment CB-03: bend radius at the machine entries
B. Akande read the failure curve from the first month, and the room saw the common cause. The cables left the tray and turned down into each machine through a plain cut-out, with the tray flange edge as the only guide, and two servo cables failed at the machine entry where the bend radius collapsed below the cable’s five times diameter rule. The rated bend radius for the heavy servo cable, 19 mm diameter, is 95 mm, but the as-built entry turned it through about 40 mm radius at the flange. The fix that kept the tray footprint was a radiused drop-out fitting at each entry with the flange cut back and a roller guide at the corner, so every cable leaves the tray on a radius at or above its datum, and a spare drop-out beside each live one for the future run.
5. Review comment CB-04: spare capacity is a design value, not a hope
C. Rossi asked the room to fix the spare rule in the minutes, because the 140-to-212 drift was not a surprise, it was the result of no rule. The drafting practice on this project was to size the trays to the drawing count, and every addition after issue pushed the fill toward the limit. The review adopted the standard rule written on the board: size every main tray at forty percent fill for the current counted bundle, then add twenty percent of the tray area as planned spare, giving an effective working fill of about thirty percent at first install. For the island the corrected main tray was 400 by 100 mm ladder tray with the divider, carrying the counted 212 cables at forty percent fill, which left the spare lane above the divider free for the two machines the plant already knows will be added next year.
6. Review comment CB-05: where the signals enter the cabinet and the strain relief
B. Akande described the cabinet entry, the point where the tray ends and the cable’s life begins. The draft brought all cables into the top of the enclosure through one grommet plate, and the panel shop found that the bundle weight hung on the terminal blocks below, pulling the first three terminal rows out of line after a week. The review changed the entry to a dedicated gland plate with individual cable glands on the upper face, sized to the bundle, and a strain-relief bar inside the cabinet at the end of the tray so the weight of the cables is carried by the frame, not by the terminals. The change added 130 euros of hardware and removed the first-week re-tighten task that nobody had put in the schedule.
7. Review comment CB-06: the labeling and the future maintenance path
F. Delgado raised the problem that comes after the tray is full and closed, finding the cable that runs the third reject gate. The current island had cables numbered only at the terminal ends, and a two-hour cable trace at the previous commissioning was the result. The review made labelling part of the tray design: a printed label every 300 mm of run on every cable, a tray map drawing posted in the cabinet door, and bar-coded tags on the bundling straps at each machine entry. The cost was a label printer and an hour of picture-taking, against the two hours the first fault will otherwise take to find.
8. Disagreement: one big tray or a tray per machine row
The room spent twenty minutes on whether the island should run one 400 mm main spine or one 200 mm tray per machine row. C. Rossi argued for the single spine: one tray, one divider, one drop-out pattern, and the ninety-metre run costs less in steel and brackets than four smaller runs. F. Delgado argued for rows, because a fault in one machine row should not force the whole island to lift the bundle, and because the next two machines the plant plans would each need their own drop-out lane that the single spine had to share. The decision went to the maintenance view, which overrules the steel view on a nine-machine island: two 300 mm spine trays, one per row of four and five machines, each with its own divider, spare lane and drop-out fittings, leaving a connecting bridge tray between the rows at the cabinet end only. The steel cost rose by 220 euros, and the fault isolation gained a whole machine row of independence.
9. Risks and mitigation
Four risks were logged. Risk 1: the tray-bridge between rows creates a single point where both rows meet; mitigation is a bolted and bonded joint that is part of the earth strap, inspected at install. Risk 2: the gland plate entry can trap moisture if the tray runs above an unsealed machine; mitigation is a drip loop at every entry and a sealed plate gasket. Risk 3: the bar-coded strap tags can be lost when a cable is moved; mitigation is printed labels on the cable sheath as the primary record and tags as secondary. Risk 4: the spare lane is only useful if it stays clear, and the panel shop historically fills empty lanes with the next cable; mitigation is a red paint stripe on the spare lane with a note in the tray map.
10. Actions and owners
| ID | Action | Owner | Deadline |
|---|---|---|---|
| ACT-1 | Reroute to two 300 mm spine trays with divider and spare lane | F. Delgado | 9 August |
| ACT-2 | Fit radiused drop-out fittings at all machine entries | B. Akande | 12 August |
| ACT-3 | Install gland plate entry with strain-relief bar in cabinet | B. Akande | 14 August |
| ACT-4 | Label every cable at 300 mm intervals, issue tray map | C. Rossi | 16 August |
| ACT-5 | Mark and protect the spare lane with paint stripe | F. Delgado | 16 August |
11. Decision summary
Route the island in two 300 mm spine trays with internal dividers separating power, signal and safety traffic. Add a separate 40 x 60 mm closed duct for the safety circuit above the spines. Fit radiused drop-out fittings at every machine entry. Size the trays at forty percent fill for the counted 212 cables with a marked spare lane at thirty percent. Enter the cabinet through a gland plate with a strain-relief bar. Label every cable at 300 mm intervals.
12. Post-meeting addendum, 30 August
The two spine trays carried the 212 cables at a measured 41 percent fill with the spare lane untouched. The first servo cable failure has not recurred through the wiring of all nine machines, and a fault on the seventh machine’s reject gate was traced in eleven minutes using the tray map, against the two-hour trace the first island needed. The plant added the two planned machines to the drawing set, and both drop into the spare lane without a re-lay.
Glossary
Fill ratio: the occupied cable cross-section as a share of tray area, held under about forty percent for new installs. Ladder tray: a longitudinal-rung tray that carries cables on a ladder of rungs. Servo power cable: a shielded cable carrying PWM drive current to a servomotor. Bend radius: the minimum radius a cable may be bent without damage. Drop-out fitting: a radiused exit that lets a cable leave a tray on a controlled curve.
Appendix. The cable count table the review signed
Cable census at review: servo power 58, encoder and feedback 49, network and fieldbus 27, safety circuit 14, general control 42, auxiliary and lighting 22, total 212. Assumed average diameter 9.5 mm with sheath, giving a mid-range fill of 61 percent in the original 300 mm tray and 41 percent in the corrected twin 300 mm spines. Separation by lane: power and drives left of the divider, encoder and network right, safety in the dedicated closed duct above. Minimum bend radius enforced at entries: 5 x diameter for fixed cables, 8 x for the flexing servo tails at the machine carriage. The sign-off block carried the fill arithmetic and the spare rule, and the chair’s closing remark read: a cable tray is sized not for the cables on the drawing today, but for the cable that will be added next month, and the forty percent fill with a marked spare lane is the difference between a commissionable island and a cable landfill.
The minutes were signed by all five attendees and issued as controlled draw ing number CBL-2114, and the tray map became a mandatory page in the cabinet door pocket on every island the shop builds from this week on.
