Reduce risk of mains flashover on a marginal PCB assembly

Thunderstorms are probably of little concern to most people, but lightning has an absurdly strong tendency to destroy my equipment.  Despite robust grounding and protection measures, hardly a year has gone by without loss of televisions, phones, modems, motors, motherboards, or HVAC controls.  It's for this reason and the occasional phase loss on the three phase service that I was faced with a charred phase protection relay.

Kailn TG30s

The particular device was another chinese Ebay special: the Kailn TG30s.  The relay is a compact rail-mountable package and had performed suitably under test.  The internals were quite similar to a quick and dirty design I had sketched up before realizing that the autumnal comfort of my sustained apathy in purposelessness was more valuable than the $8 needed to buy the part from china.  Less than a week after installation, a storm rolled through and blew it to smithereens along with an unrelated compressor motor.

The relay is constructed from two boards soldered to each other at right angles with metal support straps on either side.  The mains sensing/power networks are combinations of resistive and reactive dividers in series with protection thermistors.  The capacitors and thermistors are through-hole devices placed in a very compact arrangement, though creepage distances do meet the IEC61010-1 minimum spec for reinforced insulation. 

Inside the case, there is nothing but black soot and the smell of burned epoxy.  After a bit of degreaser and soap, the failure sequence became apparent. A few observations led me to my conclusions:

• Extensive charring on the top side near the mains terminals and thermistors
• Fused traces leading to divider networks, but no caps were punctured
• Deep burning and metallizing of case adjacent to PCB near thermistor leads
• The device tripped a 30A thermal breaker nearly an entire second after the lightning strike
• PCB is contaminated with flux, solder balls and stray solder from post-line hand assembly
• Thermistor and cap leads are trimmed relatively long

Missing traces and charring down to the glass (after cleaning)

Metallization and charring of plastic case (after cleaning)

My guess is that the strike flashed over on the board near the terminal strip or thermistor leads, either between untrimmed leads or possibly along the board surface.  In a confined case, with freshly carbonized and metallized surfaces everywhere, conduction continued long after the strike impulse passed, allowing ample time for deep charring and thermal loading of the main breaker.

I should clarify that the device wasn't entirely to blame for the failure.  While inexpensive, it did get hit with a strike impulse that was also able to damage other equipment.  This venture is more about using my knowledge to add value to the inexpensive so that I can keep motors protected while not going broke (as fast).  It's a simplistic exercise in general techniques for enforcing isolation when voltages are high or when humidity or environmental contamination becomes a risk.  

• Trim and ball component leads
• Clean the board of residues and solder fines
• Apply conformal coating

When the new device arrived, I immediately popped it open and looked it over.  The build finish was similar: long untrimmed leads, no-clean flux residues and solder fines.  I clipped the component leads short and added sufficient solder to ball the joints.  After thoroughly cleaning the assembly, I brushed on a couple applications of conformal coating in the board areas associated with mains-level voltages.  Attention should also be paid to keeping the coating from encroaching closer than a few mm from the board edge, as the PCB fits into a retainer groove in its case.  I wanted to make sure that the gap between the PCB edges was sealed, so I allowed the coating to flow through from the top before coating the bottom.  In this manner, air entrapment was avoided.

Coated top side (note balled terminal strips)

New board (top) after balling and coating.  Old board (bottom) showing untrimmed leads and missing traces

The coating material I used is not a product marketed for the purpose.  I used a solution of clear Dap Sidewinder sealant diluted with xylene.  Sidewinder is (as far as I know) a modified silicone.  The cured material has some excellent properties -- and no, it's not corrosive to the assembly as an acetoxy cure RTV silicone would be. With a bit of patience and stirring, the resin can be diluted to consistencies ranging from honey to warm maple syrup.  A brushed coating is relatively self-leveling and will dry to the touch in a few hours, though a day or so should be allowed for full curing.