Thursday, July 2, 2020

What the heck happened to this Sense power monitoring module?

Figure 1:
The exterior of this Sense SM3 power sensing module.
The connections are made via barely-visible holes on the left side while a
WiFi antenna permits connectivity onto the user's wireless network.
Click on the image for a larger version.
A friend of mine had a "Sense" tm power monitoring system at his house for a couple of years.  This device works with additional software to allow a user to monitor power consumption within their house or business, potentially offering the ability to audit loads and manage their household power consumption.  It also has the ability to monitor the production of a rooftop solar, allowing another means of monitoring its production and performance.

This system and its software wasn't without its minor quirks, but it did work pretty well.

Until recently.

A couple of months ago he started getting anomalous readings from the unit - and a day or two later, it failed to provide any current readings at all but it still read the mains voltage.  Upon opening his breaker panel he could detect the strong smell of burnt glass-epoxy circuit board so he knew that the unit had catastrophically failed in some way.

Figure 2:
The other end of the Sense unit showing the model number.
While masked for this picture, it appeared to be a
rather early production unit with a very low
serial number.  It would be interesting to know if that
fact was significant to this event.
Click on the image for a larger version.
He sent it in to the manufacturer to check about a repair and after a pandemic-induced delay of a monitor or two they finally got to looking at it and deemed it "Not economical to repair" with a comment about lightning damage;  They did offer to send him a refurbished unit for about the same price as one could get a new one for on sale, so he opted to have it sent back to him in the (unlikely) hope that a more "courageous" repair would be possible.

Thus, it landed on my workbench.

As it was, I could hear parts rattling about - almost never a good sign - and after using the "spudging" tool to get it apart I could see the problem:  Two arrays of incinerated 39 ohm surface mount resistors.

Lightning damage?  I think not!

Based on a cursory overview, this unit appears to directly rectify the 240 volt mains and apply it to a switch-mode converter - and this portion of the circuity appeared to be relatively undamaged - a fact borne out by the owner who said that it was still reporting mains voltage when he pulled it from service.  What appeared to be "smoked" were the shunt resistors for both sets of CTs (current transducers) - and the question came up:  "How the hell did that happen?"

Figure 3:
The damage - while significant - did not appear to be "total":  Had I an exemplar from which to work I could have probably repaired this thing fairly easily - but one wasn't on hand and the circuit board traces were too-badly damaged to, uhmm, trace.
Click on the image for a larger version.

Lightning damage or a power line transient causing damage/failure of the affected components seems unlikely considering the very nature of how CTs are connected and used:
  • First off, CTs are completely isolated  (galvanically) from the current-carrying conductor that they are measuring, so some sort of "arc 'n spark" of mains voltage to the sensor input would seem to be out of the question.  I would expect that the stand-off voltage of the CT on the piece of wire that was being monitored would be in the high kilovolt-range - and if there had been enough voltage to break down the insulation not only would there be visible evidence.
  • This damage appears to be a result of a longer-term fault than a brief transient, having occurred over enough time to thoroughly heat and char the board as seen in the pictures.  A very brief, high-energy transient would likely have blown components clear off the board and, at the very least, physically damaged other components in the signal path.
  • He has a "whole house" surge suppressor installed - a "good" one:  Certainly that would have suppressed a transient capable of causing direct damage via the CT input - assuming that it was likely at all.  Had a massive transient actually happened, one would expect that the suppressor would have shown signs of "distress".
  • An event capable of this sort of damage - again assuming a transient - would have surely caused other damage to something - anything - in the house:  This was not the case.
  • He has several grid-tie solar inverters at his house.  At the time of damage, these would have surely registered a transient event, had their been one.
  • Considering the time of year, the location, and the weather involved at the time this failed, the probability of lightning falls into the "bloody unlikely" category - particularly since the weather was fine in the day or two that it took for it to go from "sort of working" to "failed" status.

What was interesting was that the circuitry associated with both CTs - the one monitoring the mains, and the one for monitoring the solar - were similarly damaged, although the former appeared to be suffering far worse in terms of board damage.  As can be seen from the pictures, the damage is thermal, confined entirely to the area around the 39 ohm resistors.
Figure 4:
The most badly damaged of the set of sense resistors.
(Yes, pun intended!)
Click on the image for a larger version. 

So, what happened?

At this point, it's really not possible to be completely sure, but it looks as though there may have been either a fault in both CTs (but how likely is that?) and/or there was a deficiency in the design of the monitoring board.

What are CTs?

CTs (current transducers) are nothing more than simple transformers:  One passes the wire to be monitored through the middle of a toroidal core and a voltage is induced on the many windings of the secondary wound around it:  The current through the wire in the middle is directly proportional to the (lower) current that flows and the way this is typically done is to terminate the secondary winding with a resistance.  By using Ohm's law and measuring the voltage across that resistance, the current on the wire can be calculated.

It is absolutely imperative that a CT be terminated with a low-ish resistance as leaving it open-circuit can develop a tremendous voltage.  But, there is a potential problem (pun intended!):  Current transducers are very nearly an ideal current source - that is, whether you simply short its output together or terminate it through even a fairly high-value resistor, the current will (ideally) be the same - but knowing Ohm's law, the higher the resistance, the more voltage drop for a given current - and the more power being dissipated in the shunt resistor(s).  Clearly, if the shunt resistance had increased, something terrible would be bound to happen.
Figure 5:
 The lesser-damaged portion.  Amazingly enough, most of
these resistors still read within 10% of their original values,
likely explaining why the system "sort of" worked - until it
Click on the image for a larger version.

What I expect happened was this:
  • The original component constituting the shunt resistance - which appears to consist of ten 39 ohm resistors in parallel (for 3.9 ohms) - may have been of marginal total dissipation rating.  Under a moderate load, it's possible that these resistors have been running quite warm and over time, they have degraded, slowly increasing in value.
  • As the value increased, the calibration would have started to drift:  Whether or not that happened here over a long period is unknown - but the owner did report that it took a couple of days for the unit to go from sending alarms about nonsensical readings to the total loss of current readings.
  • As the resistance went up, so would the power dissipation of the sense resistors.  Because CTs are essentially constant current devices, as the voltage increased, the power being dissipated by those resistors would also increase.  The original failure mode was possibly that the resistance was increasing due to these resistors running hot, the increased heat would have likely caused the previously slow-moving failure to accelerate.
  • At some point, a cascade failure would have occurred, with the voltage skyrocketing - and the current remaining constant:  This would certainly explain the evidence on the board.
Interestingly, this unit carries a 200 amp rating for the CT/unit combination - but there was never a time where this rating was ever attained:  The circuit that being monitored was on a 125 amp electrical service and the failure occurred during the early spring when no air conditioning was being used.  Additionally, the "solar" circuit - which is external to the 125 amp panel (on the "utility" side, in fact) - which could not possibly have anywhere near the same current load as the entire house - was also damaged, but the resistors were not so completely incinerated as those related to the main CT.


I got my hands on a working unit and did a bit of tracing of the circuitry and found that the wiring associated with the burnt resistors was completely different than I expected in - at the very least - the following ways:
  • These resistors are connected in series to form a single 390 ohm resistance, one end of the string being connected to each of the larger power devices visible on the board.
  • The power devices - both marked "Z0M"  followed by a "G" and "E822" and are made by ST.  Both devices test "open" with a diode test function on both the working and damaged unit, but  while they look like transistors, they are likely SCRs or Triacs.
  • The other ends of the resistors are connected across the mains - each string being connected to its own side.
  • When checking the CT inputs with an ohmmeter, I found no obvious resistive shunt - and the unit with the damage read identically to the known-good unit.
Further checking of nearby components didn't show any obviously-bad devices, seeming to indicate that the damage - both physical and electrical - was very localized to the resistor strings, so at some point I'll attempt a repair, possibly replacing the string of surface-mount resistors with larger, multi-watt 390 ohm units.

What was the problem, then?
Figure 6:
The main processor board for the unit.  The damage is
actually superficial - the board covered with smoke
residue when the sense resistors incinerated themselves.
Click on the image for a larger version.

Assuming that there was not any sort of inadequacy in the original circuit design, I'm at a loss to explain the damage to the board.

What seems to have been the issue was, in fact, stressed components on the circuit board and/or a failure of the CT itself  (or even the wrong CTs being supplied) but it seems unlikely that both CTs would have failed in exactly the same way.

Barring other information, I'm tending toward believing that a gradual degradation of the shunt resistors - possibly owing to the original components being thermally stressed under normal conditions - was a problem, culminated with a cascade failure at the end.

It would be very interesting to have a peek inside other units of the same model and revision that have been installed for a while to see if they show thermal stress related to the shunt resistors.  A quick perusal on the GoogleWeb did not immediately reveal this to be a common problem, so it is possible that this is some sort of freak incident.

Unless he decides to get another unit of the same model to replace this one and a comparison is done, we'll probably never know.

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