Compliance

What is a SELV power supply?

SELV means Safety extra low voltage.

This means that

  • the voltage at the output of the power supply is so low that is isn’t considered a safety risk (less than 60V DC or 35VAC).
  • the secondary side is isolated from the primary side by double or reinforced insulation, so that the output terminals can’t become electrically dangerous by short-circuiting to the primary side (which is connected e.g. to 230VAC mains)
  • the secondary side is isolated from earth so if there are faults in other devices connected to the same earth conductor, the output of the SELV power supply can’t get unsafe.

What does 60V DC mean in practice?

Typically the 60V value is defined as ripple-free DC. This means that the peak value of the waveform is not more than 10% higher than the maximum allowed voltage, e.g. it must not be more than 70V for a 60V system.

For 35VAC, the peak value has to be considered.

The values are typically measured across a 50kΩ resistance, refer e.g. to IEC61347-1 Annex A (the standard for safety of power supply for lighting purposes).

What does double or reinforced insulation mean?

This is an insulation that is so thick that even under extreme, abnormal conditions (like power surges due to lightning) it will not cause a discharge through the isolation.

This can be achieved by either using double insulation, which is just two layers of basic (non-reinforced) insulation, or using a much thicker type of insulation, reinforced isolation. In almost all cases, using reinforced isolation is more economical than using double insulation.

While this depends on the material, typically 0.4mm of insulating plastic foil is sufficient to count as reinforced insulation, however most standards require the insulator in use to be tested for its safety.

Can the primary side be connected to Earth?

Yes, the primary side may or may not be connected to Earth (as a third conductor). As long as the secondary side is not connected to earth, this does not affect the SELV rating, however note that this might have other implications since a power supply with primary connected to Earth is a Class I power supply whereas a power supply without any Earth connection is a Class II power supply.

There are many differences in how Class I and Class II power supplies are treated, so you need to check your applicable standards for details.

Posted by Uli Köhler in Compliance, Electronics

What is a transformer construction according to clause 26.2.4.1 of IEC 61558-1?

In transformer specifications or tests, you will often find sentences like

Construction according to clause 26.2.4.1 of IEC 61558-1

This clause refers to hermetically sealed construction by impregnation potting (even if only parts of the transformer are potted), i.e. the transformer is (partially) filled with epoxy or similar potting materials in order to prevent moisture or dust from influencing its performance.

This usually means:

  • the transformer can be smaller since the safety regulations (IEC61558-1) requires  less creepage distances compared to non-potted transformers
  • the transformer manufacturer has to perform additional tests according to IEC61558-1 (compared to non-potted transformers) to prove that the potting material provides sufficient isolation
  • the transformer is typically more expensive than an equivalent non-potted transformer since it needs to be potted in the factory and additional safety tests need to be performed on transformer specimens.

Also see What is a transformer construction according to clause 19.12.3 of IEC 61558-1?

Posted by Uli Köhler in Compliance, Electronics

What is a transformer construction according to clause 19.12.3 of IEC 61558-1?

In transformer specifications or tests, you will often find sentences like

Construction according to clause 19.12.3 of IEC 61558-1

This clause refers to isolated winding wire construction, i.e. a way of making a transformer where the isolation of the winding wires themselves is the only isolation.

This usually means:

  • that the winding wires will have multiple layers of isolation (usually two or three to satisfy safety requirements from IEC61558-1)
  • that there is no additional isolation foil since the isolation of the wound wires is sufficient to fulfil the safety requirements
  • that manufacturer will need to perform additional safety tests for each and every transformer (100% production test) to prove that the transformer is safe.

Also see What is a transformer construction according to clause 26.2.4.1 of IEC 61558-1?

Posted by Uli Köhler in Compliance, Electronics

X1/X2/Y1/Y2/Y4 impulse withstand rating voltage calculator (IEC 60384-14)

IEC 60384-14 specifies that X1/X2-rated capacitors shall be tested to withstand an impulse voltage of 4 kV (X1), 2.5 kV (X2, Y4), 8 kV (Y1) or 5 kV (Y2).

However these values only apply for a capacitance \leq 1 μF (except for Y1/Y4 capacitors). Use this calculator for X1/X2/Y2 capacitances > 1 μF!

TechOverflow calculators:
You can enter values with SI suffixes like 12.2m (equivalent to 0.012) or 14k (14000) or 32u (0.000032) as well as values with units (like 12V).
The results are calculated while you type and shown directly below the calculator, so there is no need to press return or click on a Calculate button. Just make sure that all inputs are green by entering valid values.

F

Formula:

Up = \frac{Up_{\leq 1 μF}}{\Large\sqrt{\frac{C}{1\,000\,000\frac{μF}{F}}}}

where:

  • Up is the impulse withstand voltage rating
  • C is the capacitance in Farads
  • Up_{\leq 1 μF} is the voltage rating for that capacitor class with a capacitance of \leq 1 μF:
    • For X1-class: 4 kV
    • For X2-class: 2.5 kV
    • Y1-class impulse withstand voltage is always 8 kV no matter what capacitance
    • For Y2-class: 5 kV
    • Y4-class impulse withstand voltage is always 2.5 kV no matter what capacitance

Why is the impulse withstand voltage lower for larger capacitors?

The rationale behind the derating of the impulse withstand voltage is that larger capacitances will have sufficient capacitance so that a given overvoltage doesn’t cause a large voltage spike in the capacitor.

The formula (see above) is chosen so that the energy in the capacitor:

E = \frac{1}{2}\cdot{}C\cdot{}U_p^2

is kept constant (i.e. at the same value as for a equivalent capacitor of 1 μF).

Posted by Uli Köhler in Calculators, Compliance, Electronics