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Emil Brown

SMA Sunny Boy Rapid Shutdown Box

Written August 30, 2017

SMA RSS Review

The SMA Rapid Shutdown System is a necessary evil if you are installing SMA string inverters under the US 2014 National Electric Code (NEC). I am currently installing 3 of these for my 4 string, 3 inverter rooftop system.

A couple of gotchas first: If you are using only 1 of the two channels (A & B), you MUST use A. It won't work at all if you try to use only channel B. This is not mentioned in the manual best I can tell.

Deep in the manual it says that if you are using both channels A & B, they MUST go to the same inverter. So you will need at least as many of these boxes as you have inverters.

I am not impressed by the design. While it is claimed that these are NEMA 4X boxes, they ship with 3 open holes for wiring. One of these is covered with adhesive tape. The manual provides some guidance on sealing these holes up, but it is basically limited to the type of gasket you must use (thick, rubber, not thin plastic). I think suitable gaskets should be supplied with the product, or SMA should at least provide specific recommendations of listed products that are known to work.

There are also cable whips for 4 input strings. You can actually only use 2 (1 per channel) unless you have identical arrays that you want to parallel and run off the same MPPT tracker. Since residential SMA inverters now come with as many as 3 MPPT tracking circuits, it seems to me that paralleling strings is outdated and inefficient. These whips enter the box through 4-hole cable glands that are NOT sealed with rubber gaskets similar to the ones recommended for the conduit openings. Hmmm.

I found "thick" EPDM gaskets for the outside, and will use sealing lock-nuts on the inside. I had to buy these separately from the conduit connectors and cable glands that I am using because those are very hard to find with gaskets included at all, much less the kind that SMA advises. I am sure that UL would not approve.

Furthermore, the holes are on one side of the box and the cable whips are on the other so it is impossible for all these vulnerable openings to face down when you mount it. So, if you are mounting it outside, you MUST get the holes well sealed and you MUST pray that the cable whips are well sealed at the factory in some non-obvious way.

The box is rated for only 75 degrees C (167 F). I'd be leery of installing these in a desert climate where it can hit 120 in the shade. Rooftops could exceed 75 C in Phoenix I think. These boxes should be designed for 90C, not 75C.

Inside the box is a conventional looking circuit board without a conformal coating or waterproofing of any kind. If water gets to it, its toast. It is surprisingly large and complex for what is essentially just a switch. My guess is that it has a processor and firmware as well as power electronics and relays.

Installation is fairly straightforward except for wiring the control wires. The circuit board has a stack of 2 connectors with 5 wires each. One is for incoming wires and the other is for outgoing (to the next box, if you have more than one). The plugs for these are tiny and have 5 spring loaded terminals. To wire them you need 3 hands - one to hold the plug, one to depress the spring button and a 3rd to insert the wire. And you will be doing this up on the roof, or in the attic, because the plug is not tiny enough to fit through any conduit or cord connector or lock nut that would fit the provided 1/2" hole. Essentially you need the skills of an electronic technician to do this.

On the positive side, no batteries are required since it is powered entirely by the channel A array. Evidently the box is designed to shut down your DC connection to the inverter every night as the sun sets, and re-establish the connection in the morning when the sun rises and the array provides enough power (I assume it requires very little). The "controller" is nothing but a button/switch and 2 LED indicators and requires no batteries or power source other than the 5-wire control cable. This controller is manufactured by Eaton (not SMA), and is fairly generic. Once the big red button is pressed, it locks and the switch can only be released with a key (or by taking the cover off the box:). So keep the key somewhere safe, you might need it someday.

I am nervous about installing these on my roof. I am putting them under panels, and elevated above the roof surface since I really don't trust these boxes alone to keep the electronics dry through a deluge, or buried in snow. If you can mount them inside, I'd recommend doing that. I'd bet that more than a few of these will be "bypassed by the owner" eventually.

In hindsight I wish I had looked at the Bentek system. Maybe it is better engineered.

Hopefully I can update this review once I have my system running. At this point all I really know is that the boxes do pass voltage through as long as the controller is present and the button is not pressed. Surprisingly, removing the controller's cover will enable the power to flow even if the system has been locked out.

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