Engineer’s Angle: by JD Anson
Resistance is futile.
Resistance is also dangerous when it comes to electrical connections. Some electrical resistance is by design and can be quite useful such as heating elements in a hot tub or resistors in an electronic circuit.
But unintended resistance can be harmful. Heat builds where it was not intended to be or to a point above design. The more resistance, the more heat. The more heat, the more resistance. It can feed upon itself. This can cause a loss of conductivity or even a fire.
Increased resistance can build over time, or be introduced at construction. As components age, resistance increases. At each cycle of a switch contact, a microscopic deposit is left on the contact surface.
Anyone who has worked on old car distributors has seen this illustrated on the ignition points. These contacts open and close many times in their life, and eventually enough non-conductive deposit is built up to the point of affecting the contacts and interrupting spark. Because the amperage is very small, heat does not build to a degrading point before the contacts become unusable.
On a large switchboard contactor the amperage is much larger, so the contacts are heavier duty. This keeps the deposits from building very quickly as the number of cycles is relatively very small. However, the second part of a contactor can cause premature failure. Each contactor is actuated by an electrical coil. Failure of the coil due to power spikes or overheating will cause a lack of magnetism and thus, a less-than-definite connection between the contacts. Now the contacts are beginning to electrically arc, creating more deposits and heat. Each cycle makes it worse.
Initial resistance can be introduced when poor design or installation occurs. Using components and cabling too small for the amount of current creates heat as the parts strain to pass the power being required by the consumer. Properly sized components will fail if they are not installed correctly. Installation in an environment over the temperature for which they are designed will cause resistance to increase. This then causes more heat in an ever increasing spiral.
Improper securing of the individual conductors is one of the most common reasons for heat related failure. Many installers will make the connection in a terminal and not think again about it. Improperly tightened terminals will create heat resistance. Many heating/cooling cycles over time will cause the wires to loosen as individual strands move to relieve the stress upon them by the terminal clamps.
To prevent these wires from working loose, the proper wire terminals must be used. These terminals will gather and constrict the wire strands into a movement-free form that discourages wire strands from shifting.
When connecting to a screw or post terminal, a properly sized ring terminal or fork terminal with retaining tabs must be used. These are the common crimp-on type terminals and are available in insulated, non-insulated and heat shrink styles. If inserting a bare wire into a clamp type connection such as on a contactor or breaker, it is recommended that wire ferrules be used. These are metal sleeves that are slid over the stripped end of the wire and crimped with a special tool. These tools will form the sleeve tightly to the wire in a square or hexagon shape. When installing, the ferrule sides must be square to the clamp faces. If not, the contact patch will be small and create high resistance, or the ferrule can rotate in the clamp and leave the connection loose.
Class societies require periodic inspections of main switchboards. In practice, these inspections should be extended to all the subpanels as well. The procedures include a general cleaning of dust and contaminants from the inside of the panels. Then, each connection is checked for proper torque from the main buss to the smallest relay.
In addition to these services, a thermal survey should be performed. This is a quick, non-invasive process where a thermal imaging camera is used to inspect all the components and connections, looking for hot spots where trouble may be brewing. This imaging is quick and done with the boat on-line to simulate normal loading. Questionable items are easily spotted and usually can be rectified by torqueing the connections. Failing components may also be spotted before it is too late, saving unscheduled downtime.
Whether your boat is classed or not, these procedures apply equally in order to prevent failures that can be catastrophic. Preventative electrical maintenance can save much more than money. It could save your life.
JD Anson has more than 20 years of experience as a chief engineer on megayachts. He is currently project manager at Fine Line Marine Electric (finelinemarineelectric.com) in Fort Lauderdale. Comments are welcome below.