Model 307 High Current Sourcing Milliohm Meter

Owner's Manual



This instrument produces exposed high currents as a normal part of its function. High current shorts through rings, wrist bands or other conductive apparel can cause serious thermal burns which may result in permanent injury. Operators are advised to remove all conductive apparel, especially from the hands and wrists, prior to any operation or service of this instrument.

Safety Precautions

Since this instrument provides exposed high currents as a necessary part of its operation, it's very important to maintain certain safety precautions whenever using or servicing it. Be certain you've read and understand the warning above. The high currents produced by this instrument may cause the probe ends to abruptly spot weld themselves to metal objects such as rings and cause a very rapid, searing temperature rise to occur in the object. Rings are a special hazard due both to the higher chance that they may become shorted to the probes or the system under test and because, once super heated, they may be difficult or impossible to remove, thus prolonging the time and extent of injury. However, metal wrist bands, bracelets and any other conductive apparel that could possibly come into contact with this instrument's probes or any part of a unit it is testing are also hazardous and should always be removed before any operation is begun.

Always keep in mind that this instrument can behave as if it were a modest capacity arc welder whenever a test is in progress. In addition to the personal injury hazard, inadvertent probe contact may cause damage to components, wiring, metal panels or other conductive parts. This instrument, like any other high current source, should always be treated with special caution.

Be certain to visually verify that the "test current" rocker switch is in the off (down and unlighted) position before turning the instrument on, upon the completion of any test, or anytime before handling the probes.

Whenever setting the "current adjust" control for a new or modified unit under test, always start with the control set to the full counterclockwise position ("Min." position), then turn slowly clockwise until the desired current is reached.

General Description

This instrument provides efficient and accurate stress based testing of safety ground systems to insure that they meet certain performance standards of safety certification agencies such as UL, ETL, TUV, CSA, VDE, BABT and others. It utilizes probes with built in remote sensing or, for multi-point testing or the highest precision measurements, auxiliary Kelvin sense probes. In addition to ground impedance measurements, it can provide and accurately monitor the high currents necessary to test ground system endurance as is required for ground guards on etched circuit boards or in power transformers or other components.

Functionally, the instrument provides an automatically timed source of low voltage AC current adjustable to 50 amps rms at the frequency of the power source (typically 60 Hz). This current and the resulting voltage differential at the probe tips are both measured in true rms and divided to yield impedance. The value of the current is displayed continuously on a bar graph, and may be selected to be displayed on the primary digital readout at the same time. Normally, however, the digital readout is selected to display the impedance of the device under test, with a range of 200 milliohms, and a basic specified accuracy of ± 1%.

Tests are timed according to a value set on a front panel thumb wheel switch which has a resolution of one second and a range of one hour. The set time is displayed on a second digital readout, the timer readout, between tests. During tests, this readout automatically indicates the time remaining in a test as a count down timer. A timed test may be reset anytime during the test to the full test time if desired. Also, the timer may be disabled to allow indefinite testing.

An audio alert, if enabled, will sound if the primary readout is set in the impedance mode and the reading exceeds 100 milliohms, a common certification limit. This alert is muted between tests. However, it may sound briefly at the beginning or the end of a test while readings are settling.

A modular probe system which incorporates built in remote sensing provides a variety of methods to make efficient connections to the unit under test. Any combination of two Model 100 series probes may be used, or custom probes or international power receptacle banks or other fixtures may be constructed using the Model 104 cut end probe and your custom connector or fixture. A common probe combination is the Model 101 medium alligator clip for connection to chassis points and the Model 102 which plugs into a standard IEC type power receptacle, providing an efficient means of attachment to the ground pin of that receptacle. For multiple point testing or the highest precision measurements, Model 109 Kelvin sensing probes may be used in conjunction with any other probes.


Once the safety precautions are understood and committed to, you may proceed with familiarization with the instrument and its operation.

To begin, check that the "main power" and "test current" switches are in the off position, and that the "current adjust" control is rotated to its full counterclockwise ("Min.") position. Plug one end of the power cord into the IEC power receptacle on the back of the cabinet and the other end into a power source meeting the specifications printed above the IEC receptacle.

Probe Connections:

Two probes must be connected to the instrument, whichever combination of Model 100 series or custom probes or fixture interconnects best suits your connection requirements. Each probe has a rectangular power connector and a modular sense connector on the instrument end. Slide the rectangular housings of the power connectors onto one another in a side by side fashion so that they will mate properly with the front panel connectors. It doesn't matter which one is on which side. Push the power connector pair into the front panel connector pair firmly until they snap into place. Be certain the connectors are fully seated. Then plug each of the modular sense connectors into the modular receptacles on the front panel, using the right modular receptacle for the right probe and the left modular receptacle for the left probe. Don't cross the sense lines - they must plug in directly above their own power connector for the instrument to yield full accuracy. Be certain that the measurement ends of the probes are left unconnected to each other or to anything else, that the "test current" switch is off (down), and that the "current adjust" control is rotated to its full counterclockwise ("Min.") position.

Controls, Readouts and Ranges:

You may now activate the "main power" switch. During a test, the primary digital readout will read impedance or true rms current according to the setting of the "meter mode" button located to the right side of the bar graph. The basic accuracy of either reading is ± 1% for test currents above 2.5 amperes. (Measurement accuracy is unspecified below 2.5 amperes - see the specifications section for precise limitations and details.) Current readings are accurate to over 100 amperes, but the instrument should not be operated above 50 amperes or the specified duty cycle limits due to thermal limitations of certain power components. In the impedance mode, the highest reading is 199.9 milliohms, above which the readout will over-range by displaying only a "1" digit.

The bar graph indicates true rms test current at all times. A lighted bar graph window indicates a test current between the value labeled for that window up to the value of the next window. Thus, if the 30 amp window were illuminated, the test current would be between 30 and 32.5 amps. However, the last window, labeled > 40 amps, will remain lighted for all currents higher than 40 amps, and the bar graph will display additions to the 40 amp reading beginning again from the left side. Thus if the > 40 amp and the 5.0 amp windows were both lighted, the test current would be between 45 and 47.5 amps. The bar graph is accurate from 2.5 amperes to it's upper range limit of 77.5 amperes, but again, the instrument should not be operated above 50 amperes. (Instruments with serial numbers of 0140 and lower do not support bar graph readings above 40 Arms unless they have received the Performance Upgrade Package. Click here for specific serial numbers.)

Between tests, the smaller timer digital readout will read the time set on the thumb wheel switch located just above from one second up to 59 minutes, 59 seconds. During a test, it will count down from that reading to zero, at which point the "test current" rocker switch will automatically physically switch to its off position. The timer may be disabled to allow indefinite testing by setting the left most button to the "continuous test" position. The internal time base is an accurate crystal controlled type clock. However, due to timing logic limitations, the actual test time may be up to one second less than the time set on the thumb wheel switch. It is therefore advised that the thumb wheel switch be set for one second more than the time required for your tests, and that a setting of only one second be avoided. In addition, the first and third sections of the thumb wheel switch should not be set to a number higher than five. (This would be interpreted as an illogical time entry and would produce incorrectly timed tests.)

Informal Accuracy Checks:

If both probes are terminated with alligator clips (Model 101 or 105 probes), they may be connected to the accuracy check resistor mounted on the back of the instrument at this time. Otherwise your probes may be connected to a sample product or for example the opposite ends of a power cord, depending upon your probe types. For the purposes of familiarization, set the thumb wheel switch to a short time such as one minute. Verify that the setting matches the reading on the timer readout. Check again that the "current adjust" control is in the full counterclockwise ("Min.") position, then push the "test current" switch to the "on: start test" position. The switch should light. However, the timer will not begin counting down until a test current of about 2.5 amps is flowing, so you probably won't observe a count down action yet. If you are connected to the 100 milliohm check resistor, slowly increase the test current by rotating the "current adjust" control clockwise until a 17.5 amp level is reached. (Due to the power dissipation limitation of the check resistor, don't exceed a current of 17.5 amps unless for very short periods, and never exceed 35 amps.) If you are connected to a different object capable of handling more current, you may increase the current level accordingly. The timer readout should now be counting down by seconds. Set the "meter mode" switch to the out position to indicate test current on the primary readout, and verify that the reading matches that indicated by the bar graph within specifications. Then set the "meter mode" switch to the in position to indicate impedance. .

If you are connected to the accuracy check resistor, note the digital reading. Total interface or "contact" impedance between the probe connections and the device under test will be roughly 1 milliohm*, so a reading of 99 to 103 milliohms should be expected (± 1% instrument tolerance ± 1% check resistor tolerance plus ~1 milliohm contact impedance). For a more thorough appraisal, compare your reading to the test data provided in your performance verification or calibration certificate. If your reading is > 100 milliohms, you may check the alarm function by setting the "> 100 milliohm alarm" button to the in position. The alarm should be clearly audible. Allow the test to continue until the timer reaches zero, at which point the "test current" switch should automatically switch off, the alarm should stop sounding and the timer should reset to the selected test time. You may leave the "current adjust" control set as adjusted if you'd like to repeat this exercise using the same unit under test. Otherwise turn it to its full counterclockwise ("Min.") position and then disconnect the probes from the unit under test.

* Add another 1 milliohm contact impedance for each power connector terminated probe utilized (eg Model 102 or 103).

Production Testing Setup:

Actual production testing of ground systems is performed in a similar manner. A typical example would be impedance testing of your product from its ground terminal to an exposed conductive point on its cabinet. You should choose a chassis connection point which will likely give the highest impedance reading. Set the thumb wheel switch to the test time required plus one second. Set the "timed test / continuous test" button, the "meter mode" button, and the "> 100 milliohm alarm" button to their in positions unless required otherwise. If one probe is configured with an IEC power plug type connector (a Model 102 probe), connect it to the IEC power receptacle of the product being tested. Then attach the other probe to your chassis connection point. If it isn't possible to connect or clip to this point, the probe tip will have to be firmly held in contact during tests (observe safety procedures). A typical requirement would call for a test current of 25 amps and allow a ground system impedance of no more than 100 milliohms. Starting from the full counterclockwise ("Min.") position, turn the "current adjust" control slowly clockwise until the desired current is reached on your sample product. Leave the "current adjust" control at that setting. Then stop the test or allow it to time out. You are then ready for repetitive production testing.

Production Testing:

To make a production test, verify that the "test current" switch is off, connect the probes to your product, then push the "test current" switch on. Visually monitor the impedance reading to determine if your product meets certification requirements. Also monitor the test current bar graph to insure that the test current meets requirements. When your test is complete remove the probes from your product. You are then ready to test your next product.

While it shouldn't normally be necessary to adjust the test current when testing identical products, a variance in the impedance of the ground system under test or the line voltage supplying power to the Model 307 can cause a variation of the test current. If a significant variance occurs, readjust the test current to the required value and if necessary press the "reset timer" button to continue the test.

Design and Development Applications:

This instrument may also be used for ground system analysis in design and development environments. In addition to impedance measurements, it can perform endurance tests on ground guards which separate line voltage circuit areas from other areas on circuit boards or in components. Probes configured with alligator clips (Models 101 or 105) connected to the farthest opposite ends of the guard run are usually best for these tests. A typical requirement would call for the guard run to withstand a current of 25 amps or more for at least one minute with no resulting evidence of degradation.

Floating Measurements and Overloads:

The test current and measuring circuits are fully floating with respect to ground, so it is not necessary to be concerned about corruption of measurements by single point grounding of the unit under test.

The instrument is overload protected by a thermal circuit breaker built into the "main power" switch which monitors both the line and neutral power conductors and disconnects them both in the case of an extended overload on either. If activated, the switch may be reset after a brief time by turning it back on.

Standard Remote Sensing, Precision Kelvin Sensing and Multi-Point Testing:

All probes incorporate built in sense lines which provide voltage sensing at the probe heads, thus eliminating probe cable resistance and its temperature coefficient as a measurement error source. For the most precise measurements, Model 109 Kelvin sensing probes may be used in conjunction with any other probes. These provide a separate connection for the sense lines, thus eliminating even interface or "contact" impedance as an error source. This can be an important advantage when performing precision analytical measurements, particularly when maximum resolution is required for accurate comparisons of multiple measurements.

To use the Kelvin sense probes, leave the smaller modular connectors from the power probes unconnected. Connect the Kelvin sense probes (Model 109) to the modular receptacles instead. Then connect the small Kelvin sense alligator clips to your unit under test, adjacent to but not directly touching the power probe connections. Also, be certain that the right Kelvin sense line is connected to the same side of the load as the right power probe, and similarly for the left sense line and probe.

The Model 109 probes can also be useful for certain multi-point testing situations as follows: If readings need to be taken between multiple points on a unit under test, the power probes may be connected to the furthest opposite ends of the area of interest, and then the Model 109's small alligator clips may be attached to various points within the unit. The impedance readings will indicate the precise impedance between the points to which the Kelvin connections are made, even though the test current flows beyond these points. Thus multiple location readings may be taken without interrupting a test or altering the high current connections. When using this technique, keep the right side Kelvin connection oriented toward the right side power connection and similarly for the left side connections. This technique is accurate only when the full test current flows between the Kelvin connection points. If parallel shunt paths route current away from the path between your Kelvin connection points, your impedance reading may be inaccurate, reading lower than the actual impedance. If uncertain, verify your multi-point procedure by testing in the normal manner and comparing readings.

If desired, you may very easily construct your own "Model 109" Kelvin sense probes using common modular telephone type flat cable and connectors as follows: Terminate the four conductor flat cable with a telephone type modular connector on one end (orientation of the cable doesn't matter) and solder all four conductors to a small alligator clip or other connector on the other end. A 1.5 meter length is suggested for each probe, but not required. (While the impedance of the sense system is low, it is not entirely immune to corruption from stray signal reception, including magnetic fields from the high currents in the power probes. It is therefore recommended that the sense lines be of a convenient but not excessive length.)

Configuring or Customizing Your Probes and Fixtures to Maximize Connection Efficiency:

Your instrument was shipped with your selection of two Model 100 series power probes. You may find additional probes useful if your measurement requirements change or you develop a new connection methodology. Besides offering superior accuracy, the remote sensing modular probe system provides considerable versatility, allowing you to maximize connection efficiency and address unique requirements. The Model 104 cut end probe provides a straightforward and economical means to incorporate your own special connectors or fixtures into your connection strategy. For example, this probe could be connected to a bank of international power receptaclesÝ or other special connectors constructed to meet your specific requirements. In this example the ground terminals should be connected together by a heavy bus, and the probe's power lead and both of the sense lines should be connected to the center of the bus so that the distance from the connection point to the farthest opposite ends of the group of receptacles is minimized. (Physically there are two sense lines, but they serve as a single connection electrically. The use of two lines provides redundancy for extra reliability.) The two sense lines should be connected to the same physical point - don't separate the sense lines as this could cause loop currents in the sense circuit which could compromise measurement performance.

Ý The power terminals of an international receptacle bank could be connected to your dielectric withstand test instrument as well, if you provide appropriate safety features and labeling, thus yielding a single connection fixture for performing both ground integrity and dielectric withstand testing.

"Test In Progress" Signal Option and Networking

Neither the Model 306 nor the Model 307 is capable of networking to computer systems or other communication protocol handlers. Please refer to the Model 309 for advanced automation and networking capabilities.

However, the Model 307 includes provisions which allow the addition of a simple "test in progress" output signal option. The 'signal' is in the form of relay contacts which close when a test begins and open when a test ends. The contacts are fully isolated from any reference. You could use this signal to slave other test instruments such as dielectric withstand testers to the Model 307's timer function.

If requested we can add this option to your instrument for $50 FOB * Hillsboro, OR. Please advise us if you're interested in this option.


Hypatia products are warranted to the original purchaser to be free from defects in material and workmanship under normal use and conditions for a period of two years from the date of original delivery. Hypatia Inc. will replace or at its discretion repair and return defective units within three weeks with no charge except a $10.00 shipping and handling fee per unit upon their prepaid shipment to Hypatia within the warranty period.

This warranty is void if the unit in question has been visibly damaged by accident or misuse, if the unit has been serviced or modified by anyone other than an authorized representative of Hypatia, or if any warranty seal has been broken.

This is the only warranty expressed or implied given by Hypatia. Any warranty implied under State Law shall be limited to two years from original delivery to the original purchaser. Specifically excluded from this warranty is damage resulting from acts of any deity, malicious mischief, vandalism, riots, wars, improper installation or neglect in the operation or maintenance of the unit or misunderstanding of the properties of the unit. Under no circumstances shall Hypatia be obligated for consequential or other damages of any kind or description, losses or expenses in connection with or by reason of the use of, or inability to use the unit for any reason.

This warranty provides you with specific legal rights, and you may have additional rights which vary from state to state. For example, some states do not allow exclusion of consequential damage.

Any service required for any reason after the warranty period will be performed by Hypatia upon the purchaser's request according to normal Hypatia service charges in affect at the time.

Text contents V2.4, 26 July 2000

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