Scroll Compressor Pump Down, Megohm Test & Fusite Terminals


This tip will be like an episode of Columbo; we will start with the what and who and then get to the why.

  1. Don't pump down a scroll into a vacuum.
  2. Don't run a scroll in a vacuum.
  3. Don't run a high voltage megohmmeter or Hi-pot test on a scroll (As a general rule, don't go over double the rated running volts).
  4. Don't do any megohmmeter test with a scroll under vacuum.

These points have been confirmed with Copeland (Emerson) as being on the naughty list this Christmas.

Resistance/Megohm Testing

A scroll is like any other compressor in that it has a motor and a compression chamber “hermetically” sealed inside the shell. There are many differences between a scroll and a reciprocating compressor, but let's focus on the few that are pertinent to this conversation (or at least the pertinent ones I can think of):

  • In a scroll, the motor is located on the bottom, which means that the motor is immersed in refrigerant and oil. When the compressor has been off and is cold, there can even be some liquid refrigerant in the compressor.
  • A scroll has a more compact and balanced design, as there is no need for “suspension” as in a reciprocating compressor. That results in closer tolerances/distances between the electrical components and the other metal parts.

The motor's location at the bottom is the biggest thing. Copeland states in bulletin AE4-1294 that megohm readings as low as 0.5 megohms to ground are acceptable. Besides the fact that this makes a scroll difficult to successfully meg (essentially impossible with a tool like the Supco M500 because it only reads down to 20 Mohms), it is a clear indication that a scroll compressor is running tighter resistance tolerances and a higher risk of internal arcing due to many factors.

The HVACR Learning Network has an entire course about scroll and reciprocating compressor test procedures, which you can check out HERE. It's a paid course, but you can get free access to this course with an ESCO All-Access Subscription.

Additional Resistance Considerations

Another thing to consider is the scroll will read lower ohms to ground when it is cold than when it is running due to higher refrigerant/oil density at a lower temperature, and, of course, you are generally doing a meg test when a scroll has been off, so that makes it tricky.

Some of the factors that can decrease resistance further and lead to problems are:

  • Moisture contamination
  • Free metallic particles due to copper leaching (acids), small metal pieces left from copper fabrication, or metal from compressor breakdown due to other issues such as overheating, flooding, and improper lubrication
  • Other contaminants

All of this is to point out that tolerances are tight in a scroll to begin with. Add in some extra nastiness, and you are at risk. (As a side note, you can learn all about Copeland scroll anatomy by watching a free webinar on the HVACR Learning Network.)

Pump Down 

First, many scroll compressors won't even allow you to pump them down into a vacuum. Either they are equipped with a low-pressure cut-out or some sort of low-pressure/low-compression bypass like the one shown in this USPTO drawing.


For example, Copeland AE4-1303 states: “Copeland Scroll compressors incorporate internal low vacuum protection and will stop pumping (unload) when the pressure ratio exceeds approximately 10:1. There is an audible increase in sound when the scrolls start unloading.” This internal low vacuum protection prevents the compressor from pulling down into a vacuum.

In addition to that, there are lots of threats and warnings about running a scroll while it is in a vacuum, as in if you had just evacuated the system and then accidentally turned the system on. That is a bad idea on any compressor, but it's even worse on a scroll.

Why is running a (scroll) compressor into a vacuum so bad?

The totally obvious reason is that the compressor itself isn't designed to run in a vacuum. It will overheat and fail to lubricate properly, but that isn't the only reason or even the primary reason. All of the literature mentions arcing, and I spoke to more than one tech rep who mentioned the “fusite” plug arcing or being damaged.

First, Fusite is a brand name and one of the companies in the Emerson family. So, when we say “fusite,” we are using a ubiquitous term for a sealed glass-to-metal compressor terminal feed-through. There are many different types and designs of Fusite terminals, just as there are many different types and designs of compressors. There are scroll compressors that use them; there are reciprocating compressors that use them. The ice cream truck that plays that obnoxious music driving through your neighborhood probably has one—on the refrigeration compressor.

Do certain fusite terminals short out more easily than others? I'm sure some are more susceptible than others. Is that what is going in here? Maybe, but if so, it's only part of the story.

What we do know about a scroll is that the electrical tolerances are tighter, and when electrical tolerances are tighter, there is a greater likelihood of arcing.

It's about to get really nerdy here, so if you don't care, just stop reading and go back to the very beginning, memorize the four points, and move on with your life.

I can't do that… because I'm broken.

Why is vacuum an issue?

Isn't vacuum the absence of matter, and isn't matter required for electrons to arc from one surface (cathode) to the other surface (anode)?

The answer is not really simple AT ALL, but the summary is that vacuum increases the likelihood of arcing under certain circumstances, and scroll compressor terminals inside the compressor happen to be one of those circumstances.

The first thing to remember is that while electrons do travel through matter, electromagnetic fields do not require matter to exist. In either case, we are incapable of achieving a perfect vacuum. So, no matter how deep we pull a vacuum, some molecules are still present.

I've heard some techs attribute this to the corona discharge effect, which can occur due to the ionization of particles around a high-voltage conductor. I really don't see this as being the answer both because the voltages applied are not THAT high, and corona discharge is not an arc or a short in the traditional sense, just a “loss” to the environment around the conductor and a pretty cool looking light (as well a decent Mexican beer).

My opinion (and this is an opinion, not a proven fact) is that the arcing is due to something called field electron emissions, which can result in insulator breakdown in vacuum conditions (NASA has to deal with it all the time in space because space is a vacuum).


The conclusion is that while this phenomenon can happen in ANY compressor, it is made more likely in a scroll due to tighter tolerance and “motor down” configuration. That means that doing a high voltage meg test—or any running/meg testing—under vacuum is a bad idea.

If you want to read more about FusiteCopeland scroll compressors, and a great overall guide that includes evacuation procedures, just click the links.

Nerd rant over.



Genry Garcia
Genry Garcia
12/5/16 at 01:11 AM

I’ve been puzzled by this for quite a while too. I concur with you in the field emission theory to explain the induced arcing under a vacuum. In my own assessment I’d like to believe that I’ve narrowed it down to possibly, a perceived intensifying factor of the electric field as a consequence of the excessive drop in pressure inside the compressor shell or just the loss of the potential barrier that prevents the field emission from occurring to begin with for the same reason.

    Bryan Orr
    Bryan Orr
    12/5/16 at 01:17 AM

    Wow! ? I’m speechless ?

      Genry Garcia
      Genry Garcia
      12/5/16 at 01:26 AM

      My feeling exactly when I read your post. It would be great if Emerson came out and offered any more than “is bad don’t do it”.

        Bryan Orr
        Bryan Orr
        12/5/16 at 01:45 AM

        A/C nerds of the world UNITE!

Bryan Lee
Bryan Lee
10/31/17 at 10:03 AM

Damn, this was a great article. I’m sure glad Bryan Orr is broken!

9/23/20 at 08:22 AM

Early Copeland did not have “vacuum” (lip seals) protection and not sure if currently on all models. Vacuum below 26-28″ run the risk of motor failure. I had to add low/low pressure protection (direct lockout) on units with lp delay timers to prevent failures.

How is pressure ratio calculated when one or both sides are in a vacuum?
What protection is provided by other manufacturers?

Kenneth. Napier
Kenneth. Napier
6/5/22 at 09:30 PM

You can that your link to the Copeland Document Refrigeration Manual, Part 5 is aged (1970). It states “ The vacuum pump should be operated until a pressure of 1,500 microns absolute pressure is reached—at which time the vacuum should be broken with the refrigerant to be used in the sys- tem through a drier until the system pressure rises above “0” psig.”

With refrigerant, nope, EPA 608 violation.

5/7/23 at 10:18 AM

I seached the entire internet for studies related to the matter but its hard to find niche info. I found that It is possible for a winding to arc in a vacuum under certain circumstances, as the lower pressure in a vacuum can reduce the distance needed for electrical breakdown to occur. This is because the mean free path of the electrons increases in a vacuum, which can lead to a higher likelihood of arcing compared to a situation with normal atmospheric pressure.

However, it’s important to note that the presence of a refrigerant gas or any other insulating medium can also influence the likelihood of arcing. The insulating properties of a refrigerant gas, its pressure, and its temperature will all play a role in determining the propensity for arcing to occur. In some cases, the presence of a refrigerant gas might provide better insulation and reduce the chances of arcing compared to a vacuum, depending on the specific gas and its properties.

Whether a winding would arc in a vacuum or in the presence of a refrigerant gas depends on various factors, such as the properties of the insulating medium, pressure, temperature, and the applied voltage. Each situation is unique, and a detailed analysis would be required to determine the likelihood of arcing in a specific scenario.

Overall i think its a generalisation to say that vaccuum causes easier breakdown, probably only in very specific and unique circumstance

Andrew Siegel
Andrew Siegel
7/14/23 at 11:41 AM

Lower pressure allows something called “avalanche breakdown” to occur at lower voltages. Google “Paschen curve” for more details. This is how neon signs and fluorescent lamps can conduct current over many feet, while the lamp terminals can safely be only a few mm apart with no arcing at all. So when you pump down the compressor, the lower internal pressure allows the remaining gas to ionize and maintain conduction through avalanche – and this “glow discharge” can quickly progress to arcing, which can overheat and crack the Fusite.


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