HW30C EMS: Adjusting the voltage meter component

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October 18, 2022, 06:51:50 pm

This post shares a recent experience Jim and I (Sue) had with our Progressive Industries HW30C EMS, which we installed in April, 2019, and with which we have had no problems…until recently.

Three weeks ago, on the morning of our 6th day at a campsite with an electrical hookup (Morro Bay SP), our EMS repeatedly shutoff power to the coach, displaying an E3, High Voltage error. The HW30C remote display showed voltage varying from 131V to 134V. Per the documentation, the HW30C shuts off power whenever the voltage exceeds 132V.

As an aside: In the past I have seen a few PE3 errors (Previous error: High Voltage), but I had not been in the coach when the EMS shut off power. Also, for those who are unaware, the HW30C remote display includes a “ByPass EMS” switch that disables all of the features of the EMS, except surge protection. Therefore, it’s possible to bypass the high and low voltage protection, however unwise that might be. We did not use that switch.

Considering the problem, I asked Jim if we should check the voltage at the pedestal with our VOM. He was dismissive. Although he did not share it at the time, he later acknowledged that he had a hard time believing that our inexpensive VOM ($30, Innova 3320) might be more accurate than our expensive and sophisticated HW30C EMS. Jim’s face = : (

I checked with the park staff and no one else was having a problem. I asked for someone to come out and check our pedestal. After waiting a couple of hours, Jim changed his mind and checked the pedestal with our VOM. Lo and behold, it showed 126V at the pedestal. Hmmm…

I checked online and found several posts on other boards about how both the HW30C and the HW50C may require calibration of the volt meter component via a potentiometer (POT) located on a board in the housing. (PI makes it near impossible to find this documentation on their site. It’s attached below.) Some board posters suggested that the potentiometer screw may move in one direction or the other over time. This struck me as possible, but rather unlikely. Adjusting the calibration was a bit unnerving given that we couldn’t be certain that our inexpensive VOM was correct, but adjust we did. In hindsight, for confirmation, we should have asked a park maintenance person to use their VOM to measure the voltage at the pedestal.

With some trepidation, we unplugged from the pedestal to shut off shore power, opened up the EMS’s front panel, plugged back in, and tested line voltage inside the EMS with our VOM. It was ranging from 124v to 126v. The EMS display showed 132v to 134v. Hmmm…

Using the documentation I found online as a guide, we adjusted the POT screw to match the reading on our VOM. See the attached photo extracted from our installation manual. I modified it to add the red text and yellow arrow to mark the location of the tiny calibration screw. Jim adjusted the POT screw while I reported voltage on the EMS display. Note that he turned the screw counter clockwise to decrease the voltage value displayed (clockwise would increase the value). Once the display matched the VOM’s reading for line voltage (again see image for line voltage L1 and L2), our adjustment was complete. Power was restored to the coach. Voilà.

We unplugged again, reinstalled the cover, plugged back in, notified park staff of the non-problem, and enjoyed power for the rest of the trip.

Since returning home, Jim took our VOM (and an identical second that we keep at home) to a lab with VOMs known to be accurate. Measurements were made at a standard 120V duplex receptacle in the lab.
   Reference VOM 1 (Fluke 87V)   = 120.0
   Reference VOM 2 (Fluke 87V)   = 120.8
   Traveling VOM (Innova 3320)   = 119.9
   Home VOM (Innova 3320)   = 120.0

Today’s measurements at home.
   The HW30C reports: 120 to 121
   With the VOM
at an inside duplex receptacle: 121.9
at a duplex receptacle in the pedestal: 122.2
   Please note, our electrical pedestal at home has the following amperage outlets: 20amp, 30amp and 50amp. We were plugged into the 30amp outlet. The check at the pedestal was performed at the 20amp outlet.

*CAUTION* Those who know a lot about electricity will see that I and Jim do not know a lot about electricity. We know enough to be dangerous. Had our Innova VOM been the instrument in error, recalibrating the EMS might have resulted in some significant problems for ourselves and our Lazy Daze. 😲

Sue and Jim

Here are Jim’s notes to self.
1.   Carry a VOM you can trust.
2.   Consider testing your VOM against a certified calibrated VOM every x years? 1? 3?
3.   If you ever install another EMS, check the accuracy of the voltage measurement component during installation and adjust it, if needed.
4.   Know and understand the electrical accuracy specification of your EMS and VOM.
Spec for HW30C: unknown
Spec of the Innova 3320
   ±(1.2% of reading + 5 digits)
   For 120V: +/- 1.9V (1.44 + 0.5)
   For 130V: +/- 2.1V (1.56 + 0.5)
Spec of a Fluke 87V
   ±(0.7 % + 2) true-rms
   For 120V: +/- 0.9V (0.84 + 0.02)
   For 130V: +/- 0.9V (0.91 + 0.02)

What does ±(x.x% of reading + x digits) mean?

Here is a simple explanation from Fluke: https://help.fluke.com/hc/en-us/articles/360060281651-In-the-accuracy-specification-1-of-reading-3-counts-what-does-the-counts-mean-

Here is a more detailed explanation from Weschler: Calculating Meter Accuracy | Meter Accuracy Specifications