So you've discovered you need EMC testing. Whoops.

The prototype works. You are close enough to production that somebody asks for the EMC evidence. Nobody has budgeted for it and the obvious reaction is to book a lab and hope the design passes.

That can be an expensive way to discover a design problem. Before buying formal testing, establish what the product actually needs and whether a pre-compliance session can expose the likely trouble while changes are still cheap.

First find out what testing you actually need

Give a compliance specialist a concrete product, not a category. Include the intended markets, power supply, radios, enclosure, connected cables and accessories, installation environment, and the hardware and firmware configuration you expect to ship.

In Australia, ACMA's evidence requirements vary by compliance level. Level 1 EMC equipment does not require testing, level 2 requires testing, and level 3 requires testing by an accredited testing body. The point is not to memorise the levels: it is to establish the applicable route before purchasing a generic 'EMC test'.

Keep EMC, electrical safety, radio and other market requirements separate. A useful scoping engagement should tell you what applies to this product and what evidence is needed for each obligation.

Buy a pre-compliance session with engineering time

Pre-compliance testing exists specifically to find likely EMC problems before full-scale testing. UL describes it as an early-design activity intended to identify the sources of likely failures before formal compliance work.

That is useful only if the engagement lets you learn something. Ask what happens when the product fails a measurement. Can your engineer attend? Is troubleshooting time included? Can you move a cable, change a filter, swap a power supply or try a board modification and measure again? How is extra time charged?

A laboratory that is excellent at running formal tests is not necessarily selling the engineering investigation you need. Buy those as separate things if necessary.

We have a working prototype and need to establish the applicable EMC requirements before production. Please quote separately for scoping, a pre-compliance assessment with troubleshooting time, and the formal testing/evidence required afterwards. Identify the product samples, operating modes, cables, accessories and documentation you need from us.

Bring the difficult version of the product

Do not spend the session testing a convenient arrangement that customers will never use. Bring the intended power supply. Bring the long cables. Exercise the interfaces. Put the processor, radios and loads into realistic operating modes. If the enclosure changes the result, test the intended enclosure or an agreed representative configuration.

For immunity work, agree how the tester can tell whether the product is still doing its job. A power LED is not enough if the actual failure mode is corrupt data, a stopped control loop or a communication link that silently disappears.

Record the exact configuration being assessed. Board revision, firmware, power supply, cables and operating mode all matter later when somebody asks whether a product change affects the existing evidence.

Use a failure to decide what needs changing

An EMC failure is a measurement, not a diagnosis. The useful next step is to identify the coupling path or operating condition that produces it, change one thing, and measure again.

The source may be the board while the thing doing the radiating is a cable. The problem may appear only when a particular power supply is used, an interface is active or a load switches. The fix might belong in the PCB, cable routing, grounding, shielding, enclosure or software-controlled operating behaviour.

Whatever works in the lab has to become manufacturable. If moving a cable solves the problem, define the routing and restraint. If a component is added, release the change properly. If an enclosure bond matters, make it part of the assembly and inspection process. 'The engineer adjusted it until it passed' is not a production specification.

Hold the commitments the result could invalidate

The value of pre-compliance work is partly in telling you which expensive decisions can now be made safely.

If the plausible remedy could change the board layout, think before ordering the production PCB quantity. If cable entry or shielding is still in play, think before committing enclosure tooling. If the power supply is implicated, do not buy a year's worth of it merely because the prototype used it.

Keep moving on work that is not threatened by the likely changes. The point is not to freeze the project. It is to avoid converting an EMC problem into scrap and rework.

Formal testing comes after the engineering question is smaller

Once the design and configuration are credible, run the formal work required for the target market. Where accredited testing is required, ACMA points Australian suppliers to NATA or its mutual-recognition partners for accredited bodies.

Keep the resulting reports with the configuration they describe. Product changes later may need assessment and record updates; do not assume either that every minor change invalidates everything or that an old report covers a substantially different product. ACMA explicitly requires suppliers to consider changes and their effect on compliance.

The useful recovery path is therefore: establish the requirements, buy enough pre-compliance engineering to find the likely problems, make the fixes reproducible, then commit the parts of production that are no longer at risk. A late compliance discovery is annoying. It does not have to turn into a blind all-or-nothing test booking.