Electrical safety

Testing of electrical safety in ultrasonic baths and homogenizers


Operators of electrical equipment have the obligation, under industrial safety regulations, the Medical Devices Act, and under accident prevention regulations, to ensure a safe operation for both users and patients. Manufacturers of electrical equipment have a strong interest in ensuring that such equipment can be demonstrably safely operated following service or repairs.
The equipment is subject to standard DIN EN 61010 and is therefore tested for electrical safety according to DIN VDE 0701/0702, following service and/or repairs.

This information is non-binding. The skilled-worker is responsible and will not be relieved by this guide.


1 What tests must be conducted?

The following tests must be conducted on the equipment in the sequence specified.

Each individual test must be concluded with a positive result before the next one begins.

If one of the individual tests cannot be conducted, the tester must decide whether the safety of the device can be nevertheless confirmed. This decision must be substantiated and documented.

If the specified threshold values are exceeded, the threshold values pursuant to product standards or manufacturer specifications shall apply.


2 Visual inspection

The equipment is inspected in order to identify any discernible exterior defects and, to the extent possible, to determine its suitability for the location of operation. Here, the following points must especially be taken into account:

– Selection and use of conductors and jacks in accordance with the intended use
– Damage to insulation
– Damage to the connecting cables
– Conditions of the mains plug, connection terminals and connection wires
– Defects on the bend protection
– Defects on the strain relief of the connecting line
– Condition of the fastenings, cable holders, fuse holders accessible to the user, etc.
– Damage to the housing and protective covers
– Signs of overload or of improper use/operation
– Signs of unauthorised interventions or modifications
– Certainty of unauthorised, detrimental soiling, corrosion or ageing
– Dirt or clogging of ventilation holes and slots
– Condition of air filters
– Tightness of containers for water, air or other media, condition of pressure control valves
– Operation of switches, control devices, setting devices, etc.
– Readability of all safety-related inscriptions or symbols, rated values and position indicators


3 Measurement of the protective conductor resistance

The protective conductor resistance is measured to verify a proper connection between the protective conductor's attaching point on the device and every contact part connected to the protective conductor that could become live in case of fault.

In order to assess the mains power cable's protective conductor, the entire length of the cable must be moved during the measurement. If changes in resistance are observed during its movement, it must be assumed that the protective conductor is damaged or that an adequate connection is not present.

For conductors up to 5 m in length and a rated current of up to 16 A, the resistance of the protective conductor may not exceed the threshold value of 0.3 Ohm.
The devices are not designed for longer conductors.

The test using a low DC test current offers a good method of detecting corroded connection terminals with unsafe resistance, in both possible polarities. Generally, this test yields diverse measured values or measured values that exceed the threshold value. Such defective connections are not found when using a test with high AC test current, since the corrosion layer is temporarily destroyed by the high test current.

During the test, the test object must be insulated against earthing and disconnected from earthed systems, in order to determine the condition of the protective conductor.


4 Measurement of the insulation resistance

This measurement may not be conducted if it has been forbidden e.g. by the manufacturer in the accompanying documents.

The insulation resistance should be measured
– between the live parts and every exposed conducting part, including the protective conductor (except for PELV);
– in the case of repair/modification, between the live parts of a SELV/PELV electric circuit and the live parts of the primary electric circuit.

During the test it is important to ensure that the device to be tested has been safely disconnected from the power supply circuit. During measurement, all switches, regulators, etc., must be closed in order to fully measure the insulation of all live parts. If required, the measurements must be conducted in multiple switch positions

Voltage-conducting parts against the protective conductor and the exposed, conductive parts connected to the protective conductor General 1.0 MOhm
Devices with heating elements 0.3 Mohm
Voltage-conducting parts against the exposed, conductive parts not connected to the protective conductor

2 MOhm

Voltage-conducting parts against exposed, conductive parts with the SELV, PELV protective circuits (e.g. input / output signal)
During the repair/modification between the live parts of a SELV/PELV electric circuit and the live parts of the primary electric circuit.
Live parts with the SELV, PELV protective circuits (e.g. input / output signal) against exposed, conductive parts that are not connected with the protective conductor

0.25 Mohm


5 Measurement of the leakage and touch currents

Every electrical device generates leakage currents that can be divided into 2 categories:

1. Leakage current:
The current that flows into the protective conductor and thus presents a risk to the user only in the first case of fault (protective conductor interrupted).

2. Touch current:
Current that flows to the earth through the user, when a conductive, exposed part that is not connected to the protective conductor is touched. (For this to happen, a massive error must occur – loss of the reinforced or doubled insulation)

In the case of medical devices, the current that flows from the housing or from the exposed, conductive parts to the ground when the protective conductor is disconnected, is also designated as touch current!
Leakage current and touch current are identical if the exposed part is connected to the protective conductor and no fault is present.

Since the leakage current adds itself geometrically from the ohmic and capacitive leakage or fault currents, it is generally not possible to draw conclusions on the condition of the insulation, based on the measured value.
Electric devices are equipped in many cases with non-linear elements or components, such that possible leakage currents may also contain portions with frequencies higher than 50 Hz. In the case of flow, these currents have a smaller effect on humans than an equal current with a frequency of 50 Hz. As a result, threshold values for the protective conductor current and the touch current that are higher than those specified in the norms – which relate to currents with 50 Hz – are permissible. Most testing units take this into consideration through use of a single-pole low-pass filter with a cut-off frequency of approx. 1 KHz.


6 Protective conductor current / Earth leakage current

Every device with a protective conductor must have its protective conductor current measured. The current may not exceed 3.5 mA, with 3-phase devices not exceeding 1 mA / KW. Certain types of devices also allow for higher threshold values. If the 3.5 mA limit is exceeded, the manufacturer specifications or the product standard will apply in case of doubt. The test current should total 16 A or at least double the rated current


7 Touch current

The current may not exceed 0.5 mA.

Measurements will be conducted on all contact points for metallic parts that are not connected to the protective conductor.


8 Functional check

The relevant device functions must be tested in accordance with the manufacturer's specifications, i.e. whether the device can be safely operated as per its intended purpose.


9 Documentation

All tests conducted must be fully documented. The documents should include the following information at a minimum:

– the measured values
– the testing unit
– date of test
– type of test
– test principles
– what was tested in particular
– test results
– assessment of any identified faults and conclusions regarding further operation
– name of tester


10 Assessment

The safety of the device must be evaluated by one or more skilled persons (generally qualified electricians) who have undergone appropriate training on inspecting the device in question. The Technical Guidelines for Occupational Safety TRBS 1203, for example, also provide detailed specifications regarding the skilled person. These guidelines can be downloaded free of charge from the German Federal Institute for Occupational Safety and Health website at http://www.baua.de.

When the safety of the test object is not present, it must be labelled accordingly and its risk level must be communicated to the operator.


11 Suitable measuring instruments and testing units

The measuring instruments and testing units used must be built in accordance with international product standards IEC 61010 and (in part) IEC 61557. Additionally, requirements in accordance with DIN VDE 0404 must be adhered to in Germany. These standards define requirements pertaining to safety, accuracy, measurement procedures and influence conditions.

There are measuring instruments and testing units that do not satisfy the above-mentioned requirements. These measuring instruments and testing units may only be used when the requirements on safety and test results can be proven to be identical.

The selection of a suitable testing unit is the responsibility of the operator and/or the assigned tester.

1.1 Testing procedure

START
Visual test
Protective conductor resistance
Insulation resistance
Leakage currents
Touch currents
Functional check
Documentation
Assessment

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