Metal Enclosure Connected to PCB Ground for ESD Reasons

[ablaye]

Hello,

I have a design where I had to connect the PCB Ground to the metal enclosure to pass ESD testing. The device gets its power from an external DC power supply (12V), therefore it is considered a secondary circuit. The device is NOT an applied part. However, I am concerned with Touch Current. The operator may come in contact with the enclosure of the device, and the patient at the same time. According to the IEC 60601-1 (Ed. 3.2) in Section 8.4.2 (b), these limits for Touch Current shall not be exceeded: 100 μA in NORMAL CONDITION and 500 μA in SINGLE FAULT CONDITION.

My questions are as follows:
* How do I address the concern that the Touch Current through the metal enclosure may exceed above limits since the PCB ground is connected to the enclosure?
* Do I really need to worry about implementing air clearance and creepage distance requirements for MOOP even though the voltage is relatively low (12V) and I have a 3A fuse?

Any help would be really appreciated.

Thank you

 

[Peter Selvey]

Normally the 12V is fully contained inside the box, so there's no plausible way for it to be a source of touch current. If it shorts to the box, it's just an internal short, not related to currents that could flow through the operator or patient. It depends on the design, but usually the box is a faraday cage and doesn't really matter what goes on inside.

 

[ablaye]

Touch Current is defined in the IEC 60601-1 Standard as:

LEAKAGE CURRENT flowing from the ENCLOSURE or from parts thereof, excluding PATIENT
CONNECTIONS, accessible to any OPERATOR or PATIENT in NORMAL USE, through an external path
other than the PROTECTIVE EARTH CONDUCTOR, to earth or to another part of the ENCLOSURE

The metal enclosure which the operator may come in contact with is subject to leakage current requirements.

 

[Peter Selvey]

Yes, you need to measure the touch current from it, but the underlying question or analysis is, what are the voltage sources that can potentially create touch current above the limits and then what prevents the touch current from exceeding the limits.

Touch current is in effect the current through a 1kΩ resistor. There are two sides to the resistor. So the question for 12V inside a metal box, how do you get into a situation where the 12V causes the current to exceed the limits. If it's fully enclosed, then the 12V can't cause the touch current to exceed the limit and hence doesn't require any MOP.

You can still get touch currents from the metal box, but they are more likely to be leakage sourced from mains voltage, via EMC caps in the power supply. So it's still important to measure, but the MOP will be in the separate mains supply, not the box, and definitely not related to the 12V inside the box.

However not all designs are a simple fully enclosed metal box. If you can think of a way in normal or single fault that the 12V can be accessed in such a way as to make current flow through the 1k resistor, it may require insulation/MOP.