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I am in the process of building a small off-grid solar setup.

There will be 4 parallel strings of panels in this setup, and I understand that for 3 or more strings, you need some sort of over-current protection to prevent back-current in the case of a short.

However I've read in several places that it is 'good practice' for floating DC power supplies to have DOUBLE pole breakers (ie on positive AND negative) if they do not have their negative terminal grounded ie they are 'floating'. I have specifically seen this referenced in discussion and standard about PV panels.

I don't understand why - surely with one breaker on the positive side tripped, there is no circuit and the panel/supply is isolated?

Can someone please explain the specific mechanism, in plain terms, as to why and why a double pole breaker is needed in a PV array? What fault/scenario are double-pole breakers protecting against?

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    \$\begingroup\$ Depends on the legislation where you live and what peak voltage you expect. Where do you live? \$\endgroup\$ – winny Mar 26 at 12:58
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    \$\begingroup\$ Physics doesn't vary, but safety standards do. Part of the point of safety standards is everyone doing it the same way (even if two different ways are equally safe in theory). This means that when a new person comes to work on the installation, they don't get any unpleasant surprises. \$\endgroup\$ – Jack B Mar 26 at 13:12
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    \$\begingroup\$ I understand your concern but "best practice" usually comes from legislation in electrical installations. What's your maximum string voltage? \$\endgroup\$ – winny Mar 26 at 13:23
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    \$\begingroup\$ You've asked "do I need double-pole breakers?" Answering this question may depend on your jurisdiction. You also asked "what scenario are double-pole breakers protecting against?" This seems to be a separate but related question -- perhaps they should be separated? \$\endgroup\$ – LShaver Mar 26 at 13:42
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    \$\begingroup\$ Here is a link to a discussion that explains the logic behind protecting against two system faults: forum.solar-electric.com/discussion/351722/fuses-on-both-dc-and \$\endgroup\$ – AlmostDone Mar 26 at 14:05
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Consider a short to ground on the high side, before the breaker.

If the low side is grounded (not floating) the short to ground is a dead short, and either a breaker will trip, a fuse will blow or something will melt. Either way, it'll get fixed.

If the system is floating, a short to ground on the high side doesn't affect the performance of the system. It continues to work just fine. Some time later (maybe even years later) someone wants to do some work on the system, so they flip off the breaker. But if the breaker only disconnects the high side, then the low side is left with a large negative voltage. The person gets a shock.

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  • \$\begingroup\$ Thanks, this is useful. But I'm still a bit confused. In the above scenario, would you not place any breaker/fuse on the high side, right at the source? So as to prevent the scenario you describe? If a short before the breaker WAS possible, even in a grounded system, as you said this would result in immediate catastrophic failure (ie fire). Seems like sticking in a breaker on the negative to counter a scenario based on poor positioning of the positive breaker is equally bad practice to me? \$\endgroup\$ – in2deep Mar 26 at 22:20
  • \$\begingroup\$ It may not always be possible to put a breaker right at the source. e.g. what if the short is inside the PV unit, due to water ingress? The fire risk should have been considered by the designers, and they will have taken precautions, such as designing the unit without flammable materials, and/or designing a glass front and metal back to prevent the spread of fire and/or integrated fuses (which would prevent a short starting a fire, but won't do anything to prevent a shock). \$\endgroup\$ – Jack B Mar 27 at 10:48
  • \$\begingroup\$ Good safety requires working through all the risks, and eliminating or controlling them. If you can eliminate the risk with a well placed breaker on the positive side - great. It's not usually possible though, so other controls are needed, such as the fireproof case, integrated fuses, and breaker on the negative side. Thinking through every possible risk is very difficult, which is why we develop "good practice" as a list of things we should do most of the time. If you want to deviate from that list, it's on you to make sure you've thought of everything that could go wrong. \$\endgroup\$ – Jack B Mar 27 at 10:55

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