Fieldpiece SMG5 Megohmmeter: Uses, Safe Use and Published Specs
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The Fieldpiece SMG5 is a handheld digital megohmmeter, sometimes called a megger, not a general-purpose multimeter. Fieldpiece describes it for checking the insulation of motor, compressor and transformer windings, mainly on HVAC and refrigeration equipment. It applies a 1000 V DC source and displays resistance up to 2000 megohms. If you need to measure AC voltage, DC voltage, current or continuity on everyday circuits, the SMG5 is not the right tool, and the manual does not present it that way.
Quick answer
- What it is: a standalone insulation resistance meter that outputs 1000 V DC and reads up to 2000 megohms, according to Fieldpiece.
- Who the maker says it is for: trained service personnel working on A/C and refrigeration equipment. The manual states it is not a consumer product.
- Hard limits: do not measure a compressor motor that is in a vacuum, do not use it on systems rated above the voltages in the manual, and remove power from the circuit under test first.
- Not stated: the manual does not give a CAT measurement category for the SMG5.
What an insulation resistance meter does
A standard digital multimeter sends only a small voltage through the leads when it reads resistance. The Fieldpiece manual explains that this is not enough to reveal high-resistance faults caused by moisture or other contamination. A megohmmeter instead applies a high voltage to the winding and measures the very small leakage current that flows through the insulation to ground. The resistance it displays is therefore very high, and the reading is useful for trends over time rather than as a single pass or fail number.
The manual says insulation resistance can change by a factor of two when the temperature moves from 70 °F to 100 °F. For that reason, compare readings taken at the same temperature, and compare them against the equipment manufacturer’s recommended values wherever those values exist.
Published specifications
The figures below come from the SMG5 operator’s manual (version 14) and the Fieldpiece product page. Where the two sources state the same figure, the manual is quoted.
| Item | What Fieldpiece states |
|---|---|
| Test voltage | 1000 V DC source; rated voltage listed as DC-DC converter to 1000 VDC |
| Measuring range | Up to 2000 megohms; 20 M and 2000 M ranges |
| Display | 3 1/2 digit LCD, maximum reading 1999, backlight |
| Accuracy, 20 M range | ±(2% rdg + 2 dgts) |
| Accuracy, 2000 M range | below 500: ±(4% rdg + 2 dgts); above 500: ±(5% rdg + 2 dgts) |
| Maximum current output | less than 1 mA |
| Measurement rate | 2.5 times per second, nominal |
| Power | 4 × 1.5 V AAA (UM-4 / R03) |
| Battery life | 4 hours typical with alkaline batteries, continuity test at the 20 range |
| Auto power off | about 15 seconds |
| Operating environment | 0 °C to 40 °C, below 70% relative humidity |
| Storage | -20 °C to 60 °C, 0 to 80% R.H., battery removed |
| Dimensions and weight | 170 mm × 44 mm × 40 mm; 160 g (0.35 lb) |
| Warranty | one year limited, from date of purchase through an authorized Fieldpiece dealer |
The manual includes the double-insulation symbol (protection class II) and the dangerous-voltage symbol. It does not state a CAT rating, a voltage category or an IEC 61010 category for the SMG5. Do not assume a category from the 1000 V figure, because a source voltage is not the same thing as a measurement category.
How to run an insulation test, step by step
The steps below paraphrase the Quick Start section of the Fieldpiece manual. Read the full manual before the first use.
- Disconnect the winding. Disconnect all wires from the winding you want to test. In a three-phase system, the manual says to test each winding separately.
- Connect the ground. Connect the E terminal to a known ground with the alligator clip, then insert the probe tip into the L terminal.
- Power on. Slide the middle switch to ON. Check the low-battery indicator before you start.
- Hold MEAS. Touch the probe to the test point, press MEAS, and hold it until the reading stabilises.
- Read and release. Release MEAS. The manual says the reading stays on the display for about 15 seconds.
- Power off. Slide the switch to OFF when finished.
The manual also describes range selection. If the meter is on the 20 M setting and reads OL during a test, select the 2000 M setting and try again. If it is on 2000 M and reads 19 or less, select 20 M for better resolution. The active range is shown at the bottom of the LCD.
Safety points from the manual
Insulation testing puts 1000 V on the leads, so the warnings in the manual matter more than they do for a typical meter. The manual says to:
- read and understand the full operator’s manual before use;
- remove power from the circuit under test;
- remove test leads before opening the case or battery cover, and never operate the meter with the battery cover open;
- keep fingers off the test lead tips, test points and terminals while MEAS is pressed;
- take care with the switch: the manual warns that if the switch is ON and MEAS is pressed, 1000 V appears on the terminals;
- not measure when the compressor motor is in a vacuum;
- not use the instrument in an explosive atmosphere or where flammable gas or smoke is present;
- not use it on a system whose rated voltage is higher than the manual lists;
- not use it if the cable insulation, instrument or accessories look damaged;
- use personal protective equipment when conditions require it.
The manual also says the meter is not a consumer product and that only qualified personnel trained in service and installation of A/C and refrigeration equipment should use it. If you are not trained in that work, a licensed technician should make these measurements.
How to read the results
The manual gives a general guide for compressor insulation readings. It warns that the manufacturer’s own numbers for the equipment may be different, and that results vary with temperature.
| Reading | Condition the manual describes | Action the manual gives |
|---|---|---|
| Above 100 MΩ | Excellent | None |
| 50 to 100 MΩ | Some moisture present | Change filter drier |
| 20 to 50 MΩ | Contamination or moisture present | Change filter drier several times |
| 0 to 20 MΩ | Severe contamination or moisture | Change oil if acid is present; full system clean-up and re-evaluation |
A low reading does not identify the fault by itself. The manual also describes a second check: a milliohmmeter, such as the Fieldpiece AMR1, supplies a larger current at a very low voltage and shows whether the winding current takes a short path through failed insulation. The manual presents the two checks as complementary, and this is the point where a trained technician usually decides what to do next.
Buying checklist
- Confirm the job. If the job is insulation testing on motors, compressors or transformers, the SMG5 matches what the manual describes. If the job is AC voltage, DC voltage, current or continuity, buy a multimeter instead.
- Check the range you need. The SMG5 reads up to 2000 megohms. The manual does not give a figure above that range.
- Check the accessories. The manual’s included list is a meter, two probe tips, a ground lead with a detachable tip, an alligator probe tip, a carrying case, four AAA batteries, the operator’s manual and a one-year limited warranty.
- Check the category paperwork. The manual does not state a CAT rating. Ask the seller or Fieldpiece for the measurement category before using the meter on a live system.
- Check the battery. The manual states a low-battery indication below the operating level, and says a low battery will affect very low resistance readings.
Mistakes to avoid
- Testing a winding without disconnecting it. The manual says to disconnect all wires from the winding first.
- Using it like a multimeter. The manual’s own comparison says a multimeter’s resistance function is not the right tool for high-resistance faults.
- Comparing readings at different temperatures. The manual says insulation resistance can change by a factor of two over a 30 °F change.
- Reading the display as a pass or fail result. The manual’s guide gives ranges, but it also tells you to check the equipment manufacturer’s values.
How we chose
This post covers one product named in the keyword, the Fieldpiece SMG5. The facts come from the manufacturer’s operator’s manual and the Fieldpiece pages listed in Sources. The product is compared with the multimeter category only where the manual makes that comparison itself. No other models were considered for this post.
What we could not verify
- The manual does not state a CAT rating or measurement category for the SMG5.
- The sources do not give a measured accuracy for the 2000 M range below 500 M in terms other than those printed in the manual’s specification table.
- The sources do not state how the SMG5 compares with other megohmmeters on speed, noise or durability.
- The manual does not give a price, and prices are not part of this post.
FAQ
Is the Fieldpiece SMG5 a multimeter?
No. The manual describes it as a digital megohmmeter that supplies a 1000 V source and measures insulation resistance up to 2000 megohms. It is designed for insulation checks on motors, compressors and transformers, not for everyday voltage or current measurements.
Can a homeowner use the SMG5?
The manual says it is not a consumer product and that only qualified personnel trained in A/C and refrigeration service should use it. It also warns against use on a system whose rated voltage is higher than the manual lists. A homeowner who is not trained should call a licensed technician.
Why does the SMG5 need the motor disconnected?
The manual tells you to disconnect all wires from the winding under test. If the winding stays connected to other parts of the circuit, the reading may not show the insulation of that winding alone.
What does an OL reading mean?
The manual says OL means the reading is over the range. Switch to the higher range, 2000 M, and try again. The manual’s range guidance is in the operation section.
How long does the reading stay on the display?
The manual says the reading remains on the display for about 15 seconds after you release MEAS. The meter also powers off automatically after about 15 seconds to save battery life.
Does the manual give the CAT rating?
No. Do not assume one from the 1000 V test voltage. Ask the seller or Fieldpiece for the measurement category before using the meter.
Where do the accessories come from?
The manual’s list of included items covers the meter, probe tips, the ground lead, the alligator probe tip, the carrying case, four AAA batteries, the manual and a one-year limited warranty.
Related guides
- How to set a multimeter to ohms, for the resistance function on a standard meter and how it differs from an insulation test.
- Voltage testers for home use: how to choose, for the basic checks a homeowner can do safely before calling a technician.
Bottom line
The Fieldpiece SMG5 is an insulation resistance meter for trained HVAC and refrigeration service work. Use it only after the winding is disconnected and the power is removed. For voltage, current or continuity checks on everyday circuits, use a multimeter with a stated CAT rating.
Sources
- Fieldpiece Instruments, Inc. “SMG5 Digital Megohm Meter Operator’s Manual”, version 14 (2024 copyright on the document). https://resources.fieldpiece.com/wp-content/uploads/2025/10/Opman_SMG5_v14.pdf. Opened 2026-10-27.
- Fieldpiece Instruments. SMG5 product page (“Standalone Digital Megohm Meter”). https://www.fieldpiece.com/product-tag/smg5-standalone-digital-megohm-meter. Opened 2026-10-27.
- Fieldpiece Instruments. User Manuals index. https://www.fieldpiece.com/user-manuals/. Opened 2026-10-27.
