Surface Analysis Systems and Unstable Line Voltage

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We all tend to take the stability of our electric power for granted. Some parts of the US and the rest of the world can have very unstable power, and experience frequent power outages or brownouts that cause loss of productivity and sometimes damage to the system. This is especially true in warmer months, when there can be frequent and severe thunderstorms.

This blog post will explore some UPS (uninterruptible power supply) options that are suitable for surface analysis instruments as well as SEMs and other vacuum-related systems.

Uninterruptible power supplies are rated in kilovolt-amps (kVA) so the first thing you need to do is to calculate the kVA for your system. For example, let’s say that my system draws a maximum of 30 amps at 208 VAC. Using this kVA calculator, I come up with 6.24 kVA, which means I would need a UPS with a rating of at least 6.24 kVA. The higher the kVA rating, the higher the cost of the UPS.

There are three other things to consider for the kVA rating.

The first consideration: What is the main breaker on the system rated at? On older Physical Electronics XPS and AES systems, the main breaker ranges from 30 amps up to 60 amps depending on the type of system. A 60-amp 208V kVA calculation comes out to 12.48 kVA.

The second consideration: What is the actual current draw of the system? Usually, this is much lower than the main breaker current rating. For example, a PHI 5600 XPS system may have a 50-amp main breaker and a kVA of 10.4, but the actual maximum current draw might be only 20 amps (during a bake-out), which comes out to only 4.16 kVA. So, technically, a 4 kVA UPS would be adequate for a 5600 system, except that most electrical codes require that the power connection to the system is rated higher than the main breaker. This means that a 50-amp main breaker system would need a 60-amp 12.48 kVA connection to the system. 

Before installing a UPS, refer to your local electrical codes because you may need to get a permit or have the UPS installed by a licensed electrician.

The third consideration: Do you need a battery backup or just a line conditioner? A line conditioner costs much less than a UPS with batteries, and batteries also require maintenance and occasional replacement. Batteries in the UPS provide backup power to the system. Longer backup times require more batteries, which drives up the cost and the maintenance of the UPS.

If you have only brownouts rather than a complete loss of power, then a line conditioner may be all that you need.

This paper has a lot of useful information about things to consider when researching a UPS:

The links below show UPS providers that are recommended by some of the surface analysis system manufacturers. The key things to look for in a UPS are its kVA rating and that the input voltage is single phase, 208-to-240 VAC (this depends on what system you have and what your local line voltage is).

Here is a link to APC Smart-UPS:

This website has an extensive list of UPS units and conditioners for a variety of systems:

This model is a 6 kV UPS with a single 30 amp plug on the input:

And finally, here are two more UPS providers for comparison purposes:

2 thoughts on “Surface Analysis Systems and Unstable Line Voltage

  1. I fully second the recommendation to always use full-conversion UPS. Full-conversion provides complete galvanic isolation between primary and secondary AC circuits, thus serving as isolation transformer and eliminating common-mode noise. Good low-impedance connection to building ground is needed to create new “clean” neutral for secondary AC circuit, if neutral is required by the electrical code.

    APC Symmetra LX line of UPS systems is my favorite choice for particle beam instruments, with 8kVa to 16kVa models used in six different installations so far. No commercial interest in APC – just a very happy user.

  2. Dear Randy,
    Thank you for the note.
    And I would like to add the following. In a laboratory building where many facilities are used it is important to power an instrument using its individual line+grnd directly from the main source of the building and use an online/double-conversion UPS instead of line-interactive or offline/standby ones.
    Have a nice weekend,

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