I work at company producing household appliances, and I'm looking into a way to test compliance with part 27.5 of the NEN-EN-IEC 60335-1 standard (which is derived from IEC 60335-1).
It states (emphasis theirs):
The connection between the earthing terminal or earthing contact and earthed metal parts shall have low resistance.
If the clearances of basic insulation in a protective extra-low voltage circuit are based on the rated voltage of the appliance, this requirement does not apply to connections providing earthing continuity in the protective extra-low voltage circuit.
Compliance is checked by the following test.
A current derived from a source having a no-load voltage not exceeding 12 V (a.c. or d.c.) and equal to 1,5 times rated current of the appliance or 25 A, whichever is higher, is passed between the earthing terminal or earthing contact and each of the accessible metal parts in turn.
The voltage drop between the earthing terminal of the appliance or the earthing contact of the appliance inlet and the accessible metal part is measured. The resistance calculated from the current and this voltage drop shall not exceed 0,1 Ω.
NOTE 1 In case of doubt, the test is carried out until steady conditions have been established.
NOTE 2 The resistance of the supply cord is not included in the measurement.
NOTE 3 Care is to be taken to ensure that the contact resistance between the tip of the measuring probe and the metal part under test does not influence the test results.
I thought of the following way to do this:
By switching lamps on or off, the current can be chosen, in increments of about 8.3 amps. The diode is there to ensure the voltage doesn't go above 12 volts. Test point B will be connected to the device's earthing contact/terminal, and test point A will be used to test the various earthed metal parts of the device's housing.
My questions are these:
- Is this a good way of testing compliance to this part of the standard?
- I'm thinking of using a computer PSU (ATX) to supply the 12 volts. Is there a reason I shouldn't?
The lamps are halogen ones. Good idea? (I am aware of their PTC tendencies, the presence of the ampmeter fixes that.) I figure 15AWG (or 1.5 mm²) will be sufficient for wiring all of this?
- Anything else I'm not thinking of?
- Power resistors will be used in stead of lamps (due to initial resistance being to high, and PTC behavior)
- 4mm² (or 11-ish AWG) was suggested, to prevent measurement error problems