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Performing Safe And Accurate Hipot Testing in a Production Environment

Performing a routine product safety test should not in itself represent a shock hazard to the test operator. Yet, when working with an energized circuit, an operator must be aware of the hazards involved in performing the test and take the necessary safety precautions. The National Fire Protection Association (NFPA) Standard for Electrical Safety Requirements for Employee Workplaces, NFPA 70E, stipulates that only qualified persons performing electrical work be permitted access to live parts. NFPA 70E is one of the primary standards on which the Occupational Safety and Health Administration’s (OSHA’s) 29 CFR Part 1910 subpart S Electrical Safety related Work Practices is based.

What makes an Operator Qualified?

NFPA 70E defines qualified persons in the following manner:

“A qualified person shall be trained and knowledgeable of the construction and operation of equipment or a specific work method, and be trained to recognize and avoid the electrical hazards that might be present with respect to that equipment or work method. Such persons shall also be familiar with the proper use of special precautionary techniques, personal protective equipment, insulating and shielding materials, and insulating tools and test
equipment.”

It is the employer’s responsibility to provide safety related work practices, maintain a safe working environment and train the employees implementing those practices. One way an employer can help ensure a safe working environment is by using electrical safety testers with safety agency listings. Recognizing this, OSHA requires that electrical instruments used in the workplace be listed by a Nationally Recognized Testing Laboratory (NRTL). There are a total of 18 NRTLs recognized by OSHA. To ensure that the hipot testing being performed is safe, and to comply with OSHA requirements, it is best to use a safety tester that is listed by one of these recognized testing labs.
The degree of training required for the operators performing the product safety test is highly dependent upon the set up of the
product safety testing workstation. Whenever possible, the workstation should be constructed so that there are no exposed energized circuits and so that it employs some positive means to protect the operator from coming in contact with the device under
test (DUT). When the electrical testing workstation does not employ positive protection, the operator must be trained to recognize and avoid the potential hazards. Figure 1 is an example of a workstation that employs automatic protection against direct contact with the DUT. The hooded enclosure is interlocked to the hipot.

The Dielectric Voltage Withstand Or Hipot Test

The dielectric voltage withstand or hipot test is a routine production line test that can be hazardous if theoperator is not aware of the potential hazards of the higher voltages that he/she is working with. The hipot test is the deliberate application of an excessive amount of voltage intended to stress the insulation of a DUT.

Here are 10 examples of the knowledge of a test operator should have as it pertains to hipot testing with exposed energized circuits:

1. A test operator should have a basic understanding of electricity, voltage, current, resistance, and how they
   relate to each other. A test operator should also understand conductors, insulators and grounding systems.
2. A test operator should have a working knowledge of the test equipment, the tests that are being
   performed, and the hazards associated with the tests, as well as the circuits that are being energized.
3. A test operator should understand the approach distances and corresponding voltages to which they may be exposed.
4. A test operator should be trained to understand the specific hazards associated with electrical energy.
   They should be trained in safetyrelated work practices and procedural requirements as necessary to provide protection from the electrical hazards associated with their respective job or task assignments. Employees should be trained to identify and understand the relationship between electrical hazards and possible injury.
5. A test operator should understand the three primary factors that determine the severity of electric shock, namely:
   A. The amount of current flowing through the body
   B. The path of the electrical current through the body
   C. The duration or length of time the person is exposed
6. A test operator should know that the human body responds to current in the following manner:
   A. 0.5 to 1 mA: perception level
   B. 5 mA: a slight shock is felt, a startle reaction is produced
   C. 6 -25 mA for women and 9 -30 mA for men: produces the inability to let go
   D. 30 – 150 mA: extreme pain, respiratory arrest, ventricular fibrillation and possible death
   E. 10 amps: cardiac arrest and severe burns can occur
7. A test operator working on or near exposed energized electrical conductors or circuit parts should be trained in methods of release of victims from contact with exposed energized conductors or circuit parts.
8. A test operator should understand that the test instrument is a variable voltage power source and the current will flow to any available ground path. A test operator should be aware that contacting the device under test (DUT) during the test can result in a dangerous shock hazard under certain conditions.
9. A test operator should understand that, if the return circuit is open during the test, then the enclosure of
   the DUT can become energized. This can occur if the return lead is open or the operator lifts the return lead from the DUT while a test is in process.
10. A test operator should be made aware of the importance of discharging a DUT. Lifting the high voltage lead from the DUT before the test is complete can leave the DUT charged. When you are performing a hipot test, you are testing the insulation between two conductors which is essentially a capacitor. This capacitor can act as a storage device and hold a charge
    even when performing an AC test. If  the circuit is opened at the peak of the applied voltage, the DUT could hold a charge, even under an AC test. When the test is allowed to finish and the voltage is reduced to zero, the charge is dissipated through the impedance of the high voltage transformer. Most DC hipot testers today employ an output shorting device to discharge the DUT, but the hipot must remain connected to the DUT throughout the test cycle.

This is just a partial listing of the knowledge required for a test operator  to be able to safely perform a hipot test. Many product safety testing workstations are set up for maximum productivity rather than safety. If the test station is not set up with positive
protection against direct contact, then a potentially hazardous situation can result. Even the placement of the test equipment can create a potential shock hazard. For instance, if the operator has to look away from the DUT to observe the test equipment, he/she could inadvertently contact an energized circuit, or a return probe could accidentally slip off resulting in an energized chassis.

Performing a hipot test on a DUT with exposed energized circuits can be much safer when a tester is used that offers the latest technology and safety features. Many testers today have multiple shut down circuits to disable high voltage. These testers use both
adjustable high limit and low limit current sense circuits. The high limit circuit will shut down the hipot within 0.5 seconds if the adjustable current threshold is exceeded. This commonly occurs when there is a breakdown of the DUT’s protective insulation.

A second sensing current is the low limit current sense circuit. During a normal hipot test, the enclosure of the DUT is at or near ground potential (see Figure 2). The low limit circuit monitors the test for a minimum current draw and will shut down the circuit if a minimum current flow through the DUT is not detected. This most often is the result of an open lead or the operator not making a good contact with the return lead (see Figure 3). In either of the above cases, the DUT chassis could become energized present a shock hazard for an operator coming in contact with the chassis.

Increasing Hipot Testing Safety

To offset this potentially dangerous situation, there have been many developments made to increase the safety of hipot testing. One example is the recent development of a GFI circuit into electrical safety testers. This provides operators with an even greater
level of protection from electrical shock. The GFI circuit reduces the risk of the operator receiving an electrical shock when testing an ungrounded DUT, and it can also protect operators who come into direct contact with the high voltage output of the electrical
safety tester. Most GFIs will shut down the high voltage if excess leakage current of 450:A is detected through the ground circuit. This is a high-speed shutdown circuit that disables the high voltage in less than 1 millisecond.

In order for the GFI circuit to properly work in electrical safety testing applications, the safety tester’s return lead needs to be ungrounded or floating. Having the return lead floating means that the case of the DUT, to which the return lead is normally
connected, must also be isolated from earth ground. If the return lead is disconnected or is open, then there is no path for the current to return to its source. If an operator were to come in contact with the DUT case, then he/she could complete the return path. The GFI is designed to eliminate this situation.

A GFI in an electrical safety tester works in the following manner. One point of measurement senses the current returning from the DUT through the return, while the other point of measure is return current combined with current coming back through earth ground. With a good DUT that is floating, these measurements should be almost identical since very little current should return through the ground connection. If there is a condition whereby the operator comes into contact with the high voltage circuit and completes a path to ground, the GFI will sense an excessive differential between these two points and shut down. However, if the DUT is grounded, all the current returns directly through the ground point thereby bypassing the other leg of the GFI. This results in a difference in measurement between these two points, causing a false GFI failure indication. If  you have to test a grounded DUT that will require the return to be grounded, then the GFI circuit must be manually disabled.

There are also some GFI circuits on the market that integrate so-called “smart” technology. In cases where the DUT is earth grounded, such devices will allow the “return ground sense” circuit to automatically disable the GFI circuit and the instrument operates in a grounded return mode of operation. This mode allows the user to perform their tests normally without the operator
having to manually change the instrument’s configuration. When test conditions change from grounded return back to a floating return, these “smart” GFIs will automatically enable themselves, allowing the tester to monitor the return condition of the
DUT itself without manipulation of the GFI circuits by the test operator.

Many testing applications are very versatile and production lines can quickly be re-configured to manufacture and test a wide range of different products. In some cases, these products might be grounded via a production roller platform, requiring the return of
the electrical safety tester to be grounded and on others the return may be floating. Smart GFIs ultimately provides the most effective safety protection since it is an active circuit monitoring the configuration of the return connection which automatically
sets itself accordingly. By eliminating the operator from the equation, such devices work as an effective safety circuit because they do not require human interaction that could invite operator error.

Other Steps Toward Safe Testing

The test operator should be trained in the care, use and inspection of any personal protective equipment and insulating tools required to do the job. The test operator should also perform a daily visual and functional verification test on the test equipment. This is done to certify that the equipment is functioning properly and to verify that the equipment will detect a fault condition. A common way of performing functional verification tests is the use of an external verification box or external resistor bank. While these verification procedures do perform their intended purpose, they also represent an extra piece of equipment that must be
hooked up to the safety tester.

In order to help satisfy this requirement, some companies like HARREXCO offer safety testers with a built-in self verification feature, since today’s safety testers contain microprocessor-controlled technology and software driven circuits that allow for verification to be built-in to the instrument. If available on your instrument, this verification test should be performed at
every shift change. Likewise, if personal protective equipment such as high voltage gloves are in use, then they must be
inspected before each use and electrically tested at a minimum of every 6 months. Any defects must be reported immediately
and the defective item must not be used.

Summary

Preventing against the risk of injury while performing hipot tests should be a primary concern of any manufacturer.

Fortunately, through the use of engineering and work practice controls, this risk can be greatly reduced. These controls should be automatically in place and enacted each time a hipot test is performed. A secondary precaution that can be taken is the use of safe and up-to-date electrical safety testers. Safety agency listed hipot testers ensure that the test equipment and workstation being used is safe. In addition, technology has been developed that enhances safe testing. GFI circuitry has greatly reduced the risk of operator shock, and maintaining functional checks and calibration schedules of the hipot tester are now being built into safety testers themselves. These developments, coupled with operator training and a proper work station set-up, have made performing electrical
safety tests much easier and safer for manufacturers.

Reference: www.conformity.com