Keeping water out of electrical circuits on board

The problem

Electrical conductors and enclosures containing electrical and electronic equipment need to prevent water from entering and affecting operation and possibly breaking down insulation. There are good simple rules to follow to ensure this is the case. I’ve seen many examples on board where these rules are not followed and water gets in where it shouldn’t.

So what are the simple rules to follow?

• If you need to join a cable in wet area (very common), first use a junction box with sealed glands for cable entry and exit.
• If there is no room for a j-box to joint a cable, always use a butt splice crimp with adhesive lined heat-shrink cable entry (generally three sizes are commonly available – smallest is red (0.5-1.5mm²), blue (1.5-2.5mm²), yellow 3.0-6.0mm²). Use the right crimp compression tool, not the cheaper alternative.
• Glands for cable entry should always be on the bottom or side of vertically mounted box; never on the top, as water can run down and eventually may find its way in.
• Leave a ‘drip loop’ with side entering cables so any water running down a cable will safely drip off and not run along and into the gland
• Glands are round for a reason, only ever pass a single round cable through a gland of the right size. If after tightening up the gland, the cable can still slip through you can increase the cable diameter using adhesive lined heatshrink tubing; use more than one layer if necessary.

How do I know if a junction box or enclosure is sealed against water entering it? See my blog post on ‘Ingress Protection ratings’.