Most switch panels switch unipolar: the positive line is interrupted, the ground remains continuously connected. This is the normal case and correct for most consumers. However, there are situations where exactly this is a problem - and for these, there are bipolar variants.
What the difference means technically
When switched unipolar, the consumer remains connected to the onboard power supply via its ground line, even when it is switched off. Electrically, it is de-energized because the circuit is interrupted - but it remains at ground potential.
When switched bipolar, positive and negative are disconnected simultaneously. The consumer is then potential-free: no connection to the onboard power supply, in any direction.
The difference is irrelevant for a light. For other consumers, it is not.
When bipolar becomes necessary
For metal hulls and corrosion risk. A continuously connected ground is the path through which stray currents flow. Where these currents can cause corrosion - on aluminum and steel hulls - complete disconnection is not comfort, but protection of the hull.
For consumers in humid environments. Bilge pumps, consumers in the engine compartment, anything that regularly sees water. An insulation fault in a unipolar switched circuit can lead to current continuing to flow through the ground, even though the switch is off.
For separate systems. Where two onboard power supply areas are deliberately separated from each other, a continuous ground would negate this separation.
When a standard or classification requires it. This is the case where the question is not to be weighed, but to be followed.
WarningFor metal hulls, the question of ground routing is not a detail. Stray currents are the most common cause of hull corrosion, and their effect only becomes apparent when the damage has already occurred - not before.
Monitor hull potential
Where ground routing is critical, it should be monitored. For this purpose, there are control panels that indicate whether a DC potential is present on the hull via a two-pole control button with two LEDs. If both LEDs light up when pressed, the hull is voltage-free.
Such control panels share the dimensions of the Series-200 distributors and can therefore be placed directly next to the two-pole distributors in the same panel. This is the sensible combination: bipolar switching and making the result controllable.
A check that is performed at the push of a button will also be performed. A measuring device that first has to be fetched will not.
What bipolar costs
Bipolar switching has a price, and it's not just financial:
- Space. Two contacts per circuit require more space. For the same size, bipolar means fewer circuits.
- Wiring. Each circuit needs its own return line to the panel, instead of running via a common ground. That's more cable and more work.
- Troubleshooting. Two possible interruption points per circuit instead of one.
Therefore, not everything is generally implemented bipolar, but specifically where it is needed. A mixed panel - unipolar distributors for the interior, bipolar for the critical circuits - is the usual and correct setup.
Clarify before ordering
- Hull material. Metal fundamentally changes the answer.
- Which circuits are in a humid environment? This list is usually shorter than expected and thus manageable.
- Are there separate onboard power supply areas? If so, they must not be reconnected via the ground.
- Does a standard or classification apply? Then the answer is there and not up for discussion.
- How many circuits will it be? Bipolar units have fewer circuits for the same size - this needs to be taken into account.
Typical errors
- Unipolar planned on a metal hull. The error whose consequences become visible latest and are most expensive.
- Everything bipolar by default. Costs space, cables, and circuits without being necessary everywhere.
- Number of circuits not adjusted. Bipolar units offer fewer circuits for the same size.
- Separate areas connected via ground. Undetectably negates the intended separation.
- No potential control. Without it, no one notices that the separation is no longer working.
