Friday 29 April 2022

Cross-mating of Solar DC Connectors and Fire Safety

Imagine you've just got home with this season's must-have kitchen appliance - perhaps the Nostalgia Pop-up Hot Dog Toaster ("why boil water when you can  toast?")

You can almost taste those delicious toasted hot dogs as you unpack your new purchase but you notice a warning label attached to its power cable:  "Do not cross-mate this plug with sockets of another manufacturer's brand."  When you check the plug on your Pop-up Hot Dog Toaster and the power socket in your kitchen  you can see no identifying markings to tell you which company made either of them.  Disappointed, you reach for a pan to put some water on to boil...

This is the farcical situation in which the solar industry finds itself today.

An industry estimated to be worth $180bn in sales, producing 500 million panels a year cannot agree on a standard for the connectors that are used to wire up a solar installation.

What's more - many solar installers are going about their business blissfully unaware of regulations that mean it is not permitted to connect DC connectors from different manufacturers together.

About Solar Connectors

DC Solar Connector pairs from a selection of manufacturers
(A) Ningbo Yuling  (B) Shenzen Leader Technology   (C) Staubli International   (D) Tonglin

Solar panels come with a plug and socket attached to flying leads that enable one panel to be connected to the next to create the electrical circuit.  This connector is commonly referred to as an MC4, but in many cases it cannot strictly be called this (of which more later).  

MC4 style plugs and sockets are enclosed by plastic shells.  One of the shells has two plastic fingers that pop outward to lock the two together as they are pushed together by hand.  So long as the two connectors are pushed fully together, they cannot be accidentally disconnected if the cables are pulled.  

The "4" in the name corresponds to the 4mm diameter contact pin in the plug and the "MC" stands for Multi Contact, a US manufacturing company that invented this design of connector, now owned by Staubli International AG, a multinational manufacturer of electrical connectors, fluid connectors and robotics.

So only connectors made by Staubli can really be called MC4.  However since its invention, Staubli has watched on as low-cost manufacturers have brought out copies of the MC4, or at least copies that match closely enough that a plug from one manufacturer and socket from another can be physically pushed together.  These connectors are often referred to (not least by the companies that make them) as "MC4 Compatible", which of course makes it sound like it should be fine to use them interchangeably.   

And there are dozens of these other manufacturers making these MC-4 compatible connectors. Most solar PV panel manufacturers will have a number of different suppliers listed on their technical construction file so they can choose between them based on price and availability.  Some of the larger PV module manufacturers even have their own-brand in-house DC connectors just for their own panels.

MC-4 "Compatible"

Prohibited in some legal jurisdictions - but how would you ever spot it?

But who decides whether these connectors are truly "compatible" with one another?  

The design and dimensions of domestic plugs and sockets are clearly defined in technical standards (for example CEE-7 standards for plugs used in Europe and BS1363 -first published in 1947- in the UK).  Any plug or socket can be tested against the standard and declared to meet it by an independent test laboratory.  All plugs and sockets that meet the standard are safely inter-operable.  

By contrast, there is no such standard for DC solar connectors.  Connector manufacturers can go to a test laboratory such as TUV and get certification for their product, but for the most part the only testing undertaken is that their own plug and socket work together.  With so many different manufacturers, it is impractical to have to test every plug with every other manufacturers socket.  

Even when a manufacturer tests its connectors with those of another manufacturer - who is to say that the design will remain compatible since the other manufacturer could make changes to its design in the meantime and has no obligation to tell the first manufacturer.

Staubli has published in-house research on cross-mated connectors (see page 22 of this report) and (while recognising the researchers are commercially conflicted) the reported results do make the case that it is not safe to mix connectors from different manufacturers.  After subjecting the cross-mated connections to 2,000 temperature cycles and 1,000 hours of damp heat the connector resistance increased leading to connectors overheating, which in turn can result in a mechanical failure and a DC electric arc leading to a fire. 

Concerns about fire safety and the interoperability of DC connectors from different manufacturers has led many national solar installation standards bodies to either prohibit or advise against the mixing of plug and socket from different manufacturers.  

5.9 Where plugs and sockets are mated together in any part of the PV array circuit they shall be of the same model and from the same manufacturer.

In the UK, MIS3002 - the MCS installer standard for solar PV adopts the IET Code of Practice in full.  What this means, and it seems that many people in the solar industry are unware of it, is that for MCS compliant installations in the UK cross-mated connections are prohibited.

In the Netherlands, where there has been a high level of concern at fires starting in solar installations, there is a similar requirement.  NEN1010 is the applicable standard for low voltage electrical installations.  For solar DC wiring it has the following to say:

712.526.1   The combination of plugs and sockets from different manufacturers is only permitted if both manufacturers endorse the compatibility of the plug-socket.

NOTE 1 It is recommended that each plug and receptacle combination be made by the same manufacturer.

So while stopping short of the black and white position taken in the IET guide, the requirement that both manufacturers endorse the inter-compatibility of their products effectively does the same thing if such an undertaking is not available (which it commonly is not). 

Theory and Practice

The standards may be good in theory, but practical difficulties emerge as soon as you get on site and try to make a solar installation that complies with their requirements.  As mentioned before, solar panel manufacturers might make one batch with one manufacturer's connectors and then switch over for the next batch to another manufacturer that has given a better price.  If the panels an installer buys crosses over the batch then it will include panels with mixed connectors.

To make things more difficult for the installer, the connectors all look very similar and have few distinguishing features and marks.  

In addition to joining each solar panel to its neighbour in the row, the installer must make electrical connections across distances longer than the leads that come with the solar panel.  At the very least this would include cables connecting the first and last panels back to the electrical inverter but might also include cables to join spaced-apart arrays together.  These cables are commonly made to the right length on site by the installer, with cable and connector crimped together using hand tools (the additional fire risks posed by hand-crimped DC connectors is the subject of an upcoming blog).  

Connectors at either end of the string of solar panels are more likely to be cross mated as the long cable back to the inverter is made by hand on site.

To comply with the regulations while preparing these extension cables the installer must either: 

(a) identify the brand of connectors on the panels and purchase the same connectors locally - but they are not easy to identify and often not available to buy locally, or 

(b) snip off the connector from the panel cable and replace it with a locally bought one that matches the connector to be used on the extension cable.  In this case we are replacing a factory-made crimp with a  hand crimped joint and possibly invalidating the panel manufacturer's warranty by modifying the product.  

To help installers meet these challenging requirements, some panel manufacturers will not mix connector manufacturers and guarantee that their panels only come with genuine Staubli MC4 connectors which are readily sourced locally by installers.  

These manufacturers remain in the minority and most solar panel manufacturers are producing modules with a variety of different connectors, many of which cannot be sourced locally.  For these, solar installers find themselves between a rock and a hard place if they are to follow the regulations.


This is an international problem and the international standards bodies hold the key to solving it.  The International Electrotechnical Commission (IEC) Technical Committee 82 has apparently been discussing whether to create a specification for solar connectors for many years, but there is no sign of progress in this area.  

Solar installers and their customers find themselves caught between regulators creating installation standards that are almost impossible to meet in practice and an IEC committee packed with industry experts drawn from businesses that appear to have a commercial incentive to avoid the standardisation that would resolve the problem.

The Type 2 connector for Electric Vehicle (EV) charging was originally developed by a commercial company - Mennekes as a  proprietary standard.  It has been adopted as the standard for EV chargers and the specification is now described in IEC 62196, allowing manufacturers to make inter-operable products for the safety and convenience of customers.  This demonstrates that it is possible to put aside narrow self-interest for the good of the whole industry and more quickly advance the transition to clean energy.

It is shameful that the representatives of the solar industry at IEC have not managed to do the same.
Even if the IEC committee decided to act today, given the time standards take to develop, we would be many years away from seeing compatible connectors on the market.  

In the meantime many solar installers are unknowingly making non-compliant installations and could find themselves liable if the worst were to happen.  A good initial step that could be taken quickly would be for solar panel manufacturers to unilaterally declare on their solar panel rating plate which make and model of connector it is fitted with and, if the type is not widely available in the after-market, to supply matching connectors alongside the module.  This would enable installers to comply with standards.