Sep 20, 2026

Why Your Control Panel Passes FAT but Fails on Site

A panel that passes FAT then fails on site is rarely a component fault. Most cases trace to simulated signals, a swapped field device, or bench power — all catchable before shipment.

A control panel that passes Factory Acceptance Test (FAT) and then fails after it is installed on site almost never has a component problem. In our build bay we see the same three causes repeat: the FAT exercised simulated signals instead of the real devices, a field device was substituted after testing, or the panel was verified on a clean bench supply rather than the site's actual power. All three are preventable, and all three are cheapest to catch while the panel is still with us.

Where our scope ends

To be clear about the boundary: we supply the enclosure, power supply, protection and terminals, then assemble, wire, label and functionally test the panel to your bill of materials. System design, PLC and SCADA programming, and site commissioning stay with you or your integrator. That boundary matters for this topic, because two of the three causes below sit exactly on it — and they are far cheaper to flag during the build than after the panel has shipped to another continent.

1. The FAT used simulated signals, not the real devices

On a bench, a 4–20 mA loop is driven by a signal generator. On site it is a transmitter with its own excitation, grounding and a cable run that may be 40 metres long. A panel that reads a clean 12.00 mA at the FAT can read 11.3 mA on site without a single component being faulty — the loop was simply never tested the way it will actually run.
Check before shipment: walk the I/O schedule and mark every channel where FAT used a simulator instead of the connected device. Those are your risk channels. If a real device was not available at test time, say so on the as-built sheet rather than letting the loop leave unflagged. Our wiring and labelling guide covers how we document each channel so this stays auditable: https://www.uniregal.com/resources/control-panel-wiring-and-labeling-guide

2. A field device was changed after the FAT

The second repeat offender is substitution. The FAT passes with transmitter A; on site someone fits transmitter B because A was out of stock or the spec changed. B has a different span, a different zero, or a different fault-current behaviour, and the panel that "passed" is now reading against assumptions it was never tested against.
Check before shipment: confirm the model numbers on the as-built drawing match the devices that will actually be fitted, including spares. A one-line deviation note on the drawing prevents a two-day site stoppage. For enclosures we build to your specification in carbon steel, 304 or 316 stainless, or galvanised steel, with IP65 or NEMA 4X ratings — but the rating only holds if the devices inside match the drawing that earned it.

3. The panel was tested on bench power, not site power

FAT is usually done on a regulated shop supply. Site power is different: phase unbalance, voltage sags during motor starts, harmonics from variable-speed drives, and brownouts that drop the bus below the supply's rated floor. A 24 VDC power supply that is happy at 230 V ±5% on the bench can drop out when the site bus sags to 200 V, and the panel "fails" with no fault of its own.
Check before shipment: verify the supply voltage and frequency against the site, and confirm the DC bus holds during a simulated brownout. If the panel will share a board with large motors or drives, size the power supply for the actual worst case, not the nameplate average. This is the cause we most often raise at build time, because it is invisible until the panel is energised on site.

The pre-shipment checklist

Before the panel leaves our bay, confirm:
  • [ ] Every analogue channel was tested with the real device, or is explicitly flagged as simulator-tested
  • [ ] Field-device model numbers match the approved drawing revision, including spares
  • [ ] Supply voltage and frequency match the site, including during a brownout
  • [ ] Earth bonding is measured, not assumed
  • [ ] The as-built drawing reflects every change made after the FAT
  • [ ] Cooling is sized for the enclosure's worst-case internal heat load plus sun load, not the shop ambient
  • [ ] Enclosure rating (IP65 / NEMA 4X) matches the actual installation environment

FAQ

Can you build a control panel using components we supply? Yes. You send the core devices — PLC, drives, relays — and we supply the enclosure, power supply, terminals and protection, then assemble, wire, label and FAT test the panel. Our build service is described here: https://www.uniregal.com/solutions/control-panel-build-service
Can you build just one panel, or is there a minimum order? Our cabinet minimum is five units, with a typical lead time of about 15 days; loose components ship in 3–5 days. For one-off or prototype builds, tell us what you have and we will tell you the realistic path.
Do you provide a FAT record? Yes. Every panel leaves with a functional test record against your I/O schedule. Where a channel was simulator-tested because the real device was unavailable, that is written on the record, not left for the site to discover.
Do you write the PLC or SCADA program? No. Programming and site commissioning are your scope or your integrator's. We build, wire and test to your bill of materials and flag the boundary risks above at build time.
Can the enclosure be rated for outdoor or washdown use? Yes — IP65 for indoor dust-tight use, NEMA 4X for outdoor and washdown, in carbon steel, 304/316 stainless or galvanised. Choose the rating from the actual environment, not the catalogue default. See our enclosure options: https://www.uniregal.com/solutions/control-cabinets
Can you build to a UL-listed file? We can build to a UL file on request, using UL-listed components sourced for your target market. Tell us the destination and we will route the enclosure and parts accordingly.

Talk to us before the build

Most FAT-versus-site failures are decided at the drawing stage, not on site. If you have a panel that passed FAT and then misbehaved after installation, or one you want to de-risk before it ships, send the I/O schedule and the site power spec to https://www.uniregal.com/contact and we will flag the risk channels before the enclosure is cut.