Electrical safety

Testing the electrical safety of ultrasonic baths and homogenisers #

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The Operational Safety Ordinance and the Medical Devices Act as well as the accident prevention regulations impose an obligation on the operator of electrical equipment to ensure safe operation for users and patients. The manufacturers of electrical equipment have a strong interest in ensuring that the equipment can be demonstrably operated safely after repair or maintenance. The devices are subject to the standard DIN EN 61010 and are therefore tested with regard to electrical safety according to DIN VDE 0701/0702 after repair and/or maintenance.

Which tests are to be carried out? #

The following checks must be carried out on the unit in the order given.

Each individual examination must be completed with a positive result before the next one is started.

If one of the individual tests cannot be carried out, the inspector shall decide whether the safety of the device can nevertheless be confirmed. This decision shall be justified and documented.

If the specified limit values are exceeded, the limit values according to the product standard or according to the manufacturer's specifications apply.

Test procedure #

Visual inspection #

The unit is inspected to determine any externally visible defects and, as far as possible, its suitability for its place of use, paying particular attention to the following points:

- Proper selection and use of cables and plugs
- Damage to insulation
- Damage to the connection lines
- Condition of the mains plug, connection terminals and wires
- Defects in the bending protection
- Defects in the strain relief of the connection cable
- Condition of fixings, cable holders, fuse holders accessible to the user, etc.
- Damage to the housing and protective covers
- Signs of overloading or improper application/operation
- Signs of unauthorised interference or alteration
- contamination, corrosion or ageing that inadmissibly impairs safety
- Dirt or blockages of ventilation holes and slots
- Condition of air filters
- Tightness of containers for water, air or other media, condition of pressure relief valves
- Operability of switches, controls, adjusters, etc.
- Legibility of all safety-related inscriptions or symbols, the rating data and position indicators

Measurement of the protective conductor resistance #

The measurement of the protective conductor resistance serves to verify the proper connection between the connection point of the protective conductor on the unit and any touchable part connected to the protective conductor that could become live in the event of a fault.

To assess the protective conductor of the mains connection line, the line must be moved along its entire length during the measurement. If changes in resistance are observed during movement, it must be assumed that the protective conductor is damaged or that there is no sufficient connection.

For lines up to 5 m in length and up to a rated current of 16 A, the resistance of the protective conductor must not exceed the limit value 0.3 Ohm. The units are not specified for longer lines.

A good method of detecting corroded terminals with unsafe resistance is to test them with a low DC test current in the two possible polarities. As a rule, different measured values or measured values above the limit value are found. When testing with a high AC test current, such faulty connections cannot be found because the corrosion layer is temporarily destroyed by the high test current.

During the test, the DUT must be isolated from earth and disconnected from earthed systems to safely determine the condition of the protective conductor.

Insulation resistance measurement #

This measurement must not be carried out if, for example, it has been excluded by the manufacturer in the accompanying documents.

The insulation resistance must be measured
- between the active parts and any touchable conductive part, including the protective earth conductor (except PELV);
- during repair/modification between the active parts of a SELV/PELV circuit and the active parts of the primary circuit.

During the test, make sure that the unit under test is safely disconnected from the supply circuit. During the measurement, all switches, controllers, etc. must be closed in order to fully detect the insulation of all active parts. If necessary, measurements shall be made in several switch positions

Measurement of leakage and touch currents #

Every electrical device causes leakage currents, which can be divided into two categories:

1. leakage current:
The current that flows in the protective conductor and thus only leads to a hazard for the user in the first fault case (protective conductor interrupted).

2. touch current:
The current that flows to earth via the user when touching a conductive touchable part that is not connected to the protective conductor. (For this, there must be a massive fault - loss of reinforced or double insulation).

In the case of medical devices, the current flowing from the housing or from touchable conductive parts to earth when the protective conductor is interrupted is also referred to as touch current!
Leakage current and touch current are identical if the touchable part is connected to the protective conductor and there is no fault.

Since the leakage current is geometrically the sum of the ohmic and the capacitive leakage or fault currents, it is usually not possible to conclude the condition of the insulations from the measured value.
Electrical devices are often equipped with non-linear elements or components, so that any leakage currents also contain components with frequencies higher than 50 Hz. In the event of a current flowing through them, these currents have a lower effect on people than a current of the same size with a frequency of 50 Hz. In this case, higher limit values for the protective conductor current and the touch current are permissible than those specified in the standards - based on currents with 50 Hz. Most test devices take this into account by using a single-pole low-pass filter with a cut-off frequency of approx. 1 KHz.

Protective conductor current / earth leakage current #

The current in the protective conductor must be measured on each unit with a protective conductor. The current must not exceed 3.5 mA, for three-phase devices 1 mA / KW. Certain types of equipment also permit higher limit values. If the 3.5 mA limit is exceeded, the manufacturer's specifications or the product standard apply in case of doubt. The test current should be 16 A or at least twice the rated current.

Contact current #

The current must not exceed 0.5 mA. Measurements are taken at all points of contact with metallic parts that are not connected to the protective conductor.

Function test #

The relevant functions of the unit must be tested according to the manufacturer's instructions, i.e. whether the unit can be used safely according to its intended purpose.

Documentation #

All tests carried out must be comprehensively documented. The documentation must contain at least the following information:

- the measured values
- the test instrument
- Date of the examination
- Type of examination
- test basis
- What was tested in detail
- Result of the audit
- Evaluation of the identified deficiencies and statements on continued operation
- Name of the examiner

Evaluation #

The assessment of the safety of the device must be carried out by one or more competent persons (usually electricians) who have appropriate training for the device under investigation. Detailed information on the competent person is also provided, for example, in the Technical Rules for Industrial Safety TRBS 1203 - these rules can be downloaded free of charge from the homepage of the Federal Institute for Occupational Safety and Health.

If the DUT is not safe, it must be marked accordingly and the risk it poses must be communicated to the operator.

Suitable test & measuring equipment #

The measuring and test equipment used must be built according to the international product standards IEC 61010 and (partly) IEC 61557. In Germany, the requirements according to DIN VDE 0404 must also be complied with - These standards specify the requirements for safety, accuracy, measurement methods and influence conditions.

There are measuring and testing devices that do not meet the above requirements. These measuring and testing devices may only be used if identical requirements for safety and test results are demonstrated.

The selection of a suitable test device is the responsibility of the operator and/or the appointed tester.