It Works. Is It Safe to Touch?

A prototype that functions and a prototype that is safe are two different achievements, and the gap between them is where people get hurt. The typical first build has exposed solder joints, a mains adapter with a cut cord, a lithium cell taped to a board, and a metal housing nobody has confirmed is grounded. It works on your bench because you know where not to put your hand.

The moment someone else touches it, that knowledge protects nobody. An investor or a test participant reaching around the back has no idea which surface is live. Every check below is cheap, most take minutes, and none require a certification lab.

Start With the Voltage Source

Your risk profile is set almost entirely by where the power comes from, so classify the prototype before anything else.

  • Low-voltage DC from an external listed adapter (5V, 12V, 24V). The lowest-risk architecture available. The adapter carries the mains hazard and the UL or ETL listing, and your product only sees safe extra-low voltage. Use this for every prototype you can.
  • Battery powered. No shock hazard at typical pack voltages, but lithium chemistry introduces a fire and thermal-runaway hazard instead. That is a different discipline, covered in battery pack design, and shipping any prototype containing cells pulls in the requirements in UN 38.3 testing.
  • Mains inside the enclosure. 120V AC entering your own board or a built-in power supply. This is the category that hurts people, and it is where the checks below become mandatory rather than advisable.
  • High voltage or high stored energy. Motor drives, capacitor banks, heating elements, anything above roughly 60V DC or with capacitors that stay charged after power-off. Treat these as a separate design problem with dedicated interlocks and bleed resistors.

If you can restructure the prototype to move the mains outside the box, do it. A demonstration that runs from a listed brick eliminates most of the hazard and most of the paperwork at the same time.

Five Checks Before Anyone Else Touches It

1. Protective earth continuity. Any exposed conductive part on a mains-powered unit must connect to the ground pin. Measure it with a milliohm-capable meter, not a continuity beeper: the accepted threshold is under 0.1 ohm from the earth pin to any accessible metal, and a loose star washer will read "connected" on a beeper while failing under fault current.

2. Insulation resistance. Megohmmeter at 500V DC between live conductors tied together and the chassis. Anything above 10 megohms is normal for a dry, clean assembly. A reading in the tens of kilohms means contamination, flux residue, or a pinched wire, and it means you stop and find it.

3. Dielectric withstand (hipot). Apply 1500V AC or the DC equivalent between mains conductors and chassis for one minute and confirm no breakdown and no excessive leakage current. This is the test that finds the creepage violation you designed in without noticing. Bench hipot testers are inexpensive to rent, and the test is destructive to weak insulation by design, which is the point.

4. Touch current (leakage). Measure current from accessible surfaces to earth under normal conditions and with the neutral open. General-purpose equipment limits sit around 0.5 mA; medical equipment limits are an order of magnitude tighter, which is one of many reasons IEC 60601 electrical safety testing is its own project.

5. Thermal survey under load. Run the unit at its worst realistic case (full load, enclosure closed, ambient warm, ventilation partially blocked) for at least an hour and thermocouple or thermal-image everything. Look for accessible surfaces above roughly 130F (55C) for metal, transformer and inductor cores, power semiconductors, and electrolytic capacitors sitting next to hot parts. Most field reliability failures start as a thermal problem nobody measured; the design responses are in thermal management in electronic products.

Habits That Prevent Most of the Incidents

  • Plug every mains prototype into a GFCI outlet, always. It will not stop a fault, but it will interrupt the one that would have injured someone.
  • Use an isolation transformer when probing a live board with a grounded oscilloscope. Clipping a scope ground onto a non-isolated hot node is a classic way to destroy equipment and start a fire.
  • Bring up new boards on a current-limited supply and watch the current draw before anything else. The full sequence is in PCB bring-up.
  • Never demo a unit with the lid off. Print a temporary cover if the real one is not ready.
  • Label the prototype with its voltage, and put a physical guard over anything a hand can reach.
  • Keep liquids off the bench entirely if the enclosure is not yet sealed, and check the sealing plan against your target in IP ratings before any wet demo.

How This Differs From Real Compliance Testing

Everything above is bench verification, not certification. A compliance lab working to IEC 62368-1 or IEC 61010 runs the same measurements but adds what you cannot replicate: single-fault analysis with each component deliberately failed in turn, abnormal operation tests, flammability requirements on materials, temperature rise across a full matrix of conditions, and a construction review of creepage and clearance against the tables. A listing body also audits your production line before granting a mark.

Your bench checks buy you two things: nobody gets hurt during development, and you do not walk into a paid lab session and fail on something you could have caught for free. A failed compliance run typically costs several thousand dollars and four to eight weeks of re-test scheduling. The scope of what you eventually owe is set out in which certifications a new electronic product actually needs and product safety testing requirements.

Safety Is a Design Input, Not a Final Exam

The cheapest safety work happens in CAD and schematic capture. Decide the isolation architecture before layout. Separate primary and secondary sides physically, with a routed slot where clearance is tight. Choose a listed power module rather than designing a flyback for a first product. Specify UL 94 V-0 material for any enclosure containing mains. Make the ground path a deliberate, low-impedance connection with a dedicated screw and star washer, not an afterthought bonded through paint.

Retrofitting safety into a finished prototype means moving components, respinning a board, and re-cutting an enclosure. Designing it in costs an hour of thought at the right moment.

Get a Second Set of Eyes Before the Demo

Projects House reviews and bench-tests client prototypes for electrical safety before they go in front of users, investors, or a compliance lab: grounding and isolation review, hipot and leakage measurement, thermal survey, and a written list of what to fix before certification. Describe your build and your timeline through our contact form.