I've been trying to make some sense of the newly re-written section 712 (PV) - but I'm struggling with a few things - firstly can anyone tell me what "IMOD_MAX_OCPR" represents? (e.g. in regs 712.431.101 and 102). Unlike most one-off symbols (like Np) it's not explained below, nor can I find it on page 46 (Symbols used in BS 7671). It seems to be something to do with an overcurrent protective device, but seemingly not it's nominal rating, since part of the requirement is that  In ≤  IMOD_MAX_OCPR, nor I guess breaking capacity since 1.35 IMOD_MAX_OCPR ≤ (Ns -1) ISC MAX.

   - Andy.

  • AFAIK, max OCPR=maximum over current protection rating.

    A single PV module, or a single series string of PV modules does not in practice need a fuse, though some regulations may require it.

    The short circuit current is only slightly greater than the normal load current. No fuse can reliably handle to full load current but reliably operate on the short circuit current.

    If however a number of PV modules, or a number of series strings of PV modules are paralleled, then fuses are required to allow for a faulty module or series string of modules DRAWING current from the sound modules and perhaps starting a fire.

  • AFAIK, max OCPR=maximum over current protection rating.

    thanks. So how does that differ from In? (I'm still confused)

        - Andy.

  • In AFAIK is the normal operating current of a PV module, the short circuit current is greater than the normal operating current, but only by a small margin.

  • A cut and paste into the search bar gave a selection of results:

    IMOD_MAX_OCPR is the (maximum) overcurrent protection rating of a module


  • Page 9:

    Define the specifications of the PV module


    Isc: Short-circuit current of the module at Standard Test Conditions (STC) - Data provided by the PV Manufacturer Voc: Open circuit voltage of one module at STC - Data provided by the PV Manufacturer
    Ns: Number of modules in series per string
    Np: Number of strings in parallel per array
    Imod_max_OCPR : The PV module maximum overcurrent protection rating specified by IEC 61730-2 (this is often specified by module manufacturers as the maximum series fuse rating)


  • Flicking down that Eaton pdf I came to 85 mm long D.C. fuses, which seems exceptionally long, I assume that length is required to clear a D.C. arc?

  • The length of fuses is related to the operating voltage Andy, some of the larger PV installations operate with what are really medium voltage DC outputs, leading to longer fuses to ensure that the circuit is broken properly.

    In fact these fuses are redundant in most "normal ie small household/commercial" installations because as Broadgage says the maximum output current and short circuit current are very similar.

  • I should know all about  this as I have PV C&G.

    Domestic installations rely on good engineering with over sized well insulated cables without fuses on the D.C. circuits.

  • Yes the DC and AC breaking ratings of fuses, like switches are different, for all but the smallest sizes. However if it is a supply with a current limit of the kind where the voltage collapses,  DC does not form much of an arc either, as it sort of provides its own zero-crossing. Well, not crossing exactly  but more of a dive towards zero.
    In fuses with no sand fill, like at drawn arcs, the size of the plasma fireball before is stretches to failure and breaks is broadly of the form volume is proportional to total energy delivered in the last plasma decay time, so volt drop times amps times a few msec..How many milliseconds depends a bit which materials are in the fireball but for air and a bit of copper vapour it is quite a bit less than half a mains cycle. On the other hand for welding rods things are deliberately added to the rod coating that keep enough slow cooling plasma in the mix that the arc stays conductive during the whole AC cycle, or at least  does not need a re-strike from cold 100 times a second. In a fuse you need the reverse.


  • IMOD_MAX_OCPT is the module's declared maximum current rating for the reverse current protection: Because a PV cell is not a linear device but a diode, under certain conditions (such as partial shading) the array/string/module will be operated at a voltage/current that is not available in the postive-power quadrant of the IV curve of certain strings/modules/cells and they will find themselves in reverse bias... in this case current (and power) goes the wrong way through the PV cell(s), which are also known as photodiodes. Diodes don't like this very much, hence needing overcurrent protection at string level and bypass diodes on a submodule scale.

    It probably helps to consider that historically blocking diodes were an alternative solution to string fuses. They tend not to be used these days.

    For a first, technically inaccurate, approximation this is not unlike using a mix of battery ages/specs together in series/parallel: Sometimes it'll be fine but push it too far and you might be trying to charge a non-rechargable battery which will go pop.

    Reverse current protection is not needed if you have one or two strings because if one module is in reverse bias the maximum current available is from one other string, and obviously the equipment can withstand that because that's normal load current.

    It is worth noting as it is oft forgotten that PV arrays (normally) rely on double/reinforced insulation. The fuses are not for short circuit protection. They provide overcurrent protection for the modules or cables because of the above potential for reverse current resulting in overload. Hence gPV characteristic is needed rather than gG.