Class II with Chassis earth bond?

Andrew James Skinner:

The floating output of a Class II product, if measured to Earth may present a voltage of 80-100V AC, The reason is that there is a small capacitance connected between the output side (low voltage output) and the input side (mains) of a switch mode power supply from the safety approved Y-capacitors.

The limits for this leakage current is internationally set to safe values for humans (<250uA). The Voltage is therefore regarded safe to use, and expected.


My preference in such situations therefore to isolate the electrical parts from any user accessible metal parts, to prevent  users noticing a tingle when touching them.


My questions relates to using a bond from the accessible metal parts to a safety earth instead.


1) The earth is not required for safety, only comfort but could it affect leakage current?

This depends whether the conductors of the LED strip are actually connected to the metal or not, but if it is, you might get the 250 μA as an AC. The limit for "leakage" on Class II equipment is usually about 0.5 mA in product standards.

2) The application is a pre-existing installation, an LED strip powered from a class 2 supply with the led tape mounted to a metal extrusion which itself is mounted to a metal container. If the extrusion were to be earthed to the metal enclosure, would the assembly then be treated as class I or remain class II ? 

Class I and Class II relate to the Low Voltage (mains) supply.


The term SELV (Separated Extra-Low Voltage) is used to describe an extra-low voltage source for which the output is electrically separated from the mains (in BS 7671, this is less than 120 V DC or 50 V AC in dry conditions, although some standards have lower limits for SELV, at 24 V DC and 12 V AC, so that the products may be used in environments which may become wet or damp). This is typically done by supplying it from a source similar to one of the following:

• An independent battery

• A safety isolating transformer or a source offering equivalent isolation, such as a motor-generator, opto-isolated electronics, etc

PELV is where an SELV source is used, and remains separated from live conductors (Line and Neutral) of the low voltage supply, but one conductor is connected to protective Earth.


If your LED driver is equivalent to SELV, the conductors of the tape are insulated from the metal extrusion, it remains SELV.

If one of the conductors in the tape becomes connected to Earth, then it is PELV.


If the LED driver is not adequately separated, or has higher voltages than permitted by the product standard for SELV, it is classed as FELV, more research is required, as it may be designed to provide protection by electrical separation.

3) Which standard(s) should I be referring to find these answers.

The base standard that discusses protection against electric shock, and the terms we're looking at, for both products and installations, is BS EN 61140.


The standard for LED controlgear is BS EN 61347-2-13.


The standard for many other common electronic products is BS EN 62368-1 (supersedes BS EN 60950-1). Unlike BS EN 60950-1, BS EN 62368-1 does not use the terms SELV and PELV, instead considering something called ES levels - ES1 may well be equivalent to SELV and PELV, but not always in wet conditions.

 

Andrew James Skinner:

The floating output of a Class II product, if measured to Earth may present a voltage of 80-100V AC, The reason is that there is a small capacitance connected between the output side (low voltage output) and the input side (mains) of a switch mode power supply from the safety approved Y-capacitors.

The limits for this leakage current is internationally set to safe values for humans (<250uA). The Voltage is therefore regarded safe to use, and expected.


My preference in such situations therefore to isolate the electrical parts from any user accessible metal parts, to prevent  users noticing a tingle when touching them.


My questions relates to using a bond from the accessible metal parts to a safety earth instead.


1) The earth is not required for safety, only comfort but could it affect leakage current?

This depends whether the conductors of the LED strip are actually connected to the metal or not, but if it is, you might get the 250 μA as an AC. The limit for "leakage" on Class II equipment is usually about 0.5 mA in product standards.

2) The application is a pre-existing installation, an LED strip powered from a class 2 supply with the led tape mounted to a metal extrusion which itself is mounted to a metal container. If the extrusion were to be earthed to the metal enclosure, would the assembly then be treated as class I or remain class II ? 

Class I and Class II relate to the Low Voltage (mains) supply.


The term SELV (Separated Extra-Low Voltage) is used to describe an extra-low voltage source for which the output is electrically separated from the mains (in BS 7671, this is less than 120 V DC or 50 V AC in dry conditions, although some standards have lower limits for SELV, at 24 V DC and 12 V AC, so that the products may be used in environments which may become wet or damp). This is typically done by supplying it from a source similar to one of the following:

• An independent battery

• A safety isolating transformer or a source offering equivalent isolation, such as a motor-generator, opto-isolated electronics, etc

PELV is where an SELV source is used, and remains separated from live conductors (Line and Neutral) of the low voltage supply, but one conductor is connected to protective Earth.


If your LED driver is equivalent to SELV, the conductors of the tape are insulated from the metal extrusion, it remains SELV.

If one of the conductors in the tape becomes connected to Earth, then it is PELV.


If the LED driver is not adequately separated, or has higher voltages than permitted by the product standard for SELV, it is classed as FELV, more research is required, as it may be designed to provide protection by electrical separation.

3) Which standard(s) should I be referring to find these answers.

The base standard that discusses protection against electric shock, and the terms we're looking at, for both products and installations, is BS EN 61140.


The standard for LED controlgear is BS EN 61347-2-13.


The standard for many other common electronic products is BS EN 62368-1 (supersedes BS EN 60950-1). Unlike BS EN 60950-1, BS EN 62368-1 does not use the terms SELV and PELV, instead considering something called ES levels - ES1 may well be equivalent to SELV and PELV, but not always in wet conditions.