Enclosure Grounding, Insulation

[pl510]

Hi folks,

First post here. I am working on designing a circuit board and aluminum enclosure for a lab equipment type application. I have an mains-to-12VDC enclosed AC/DC module as my primary power supply. The rest of the board runs off the 12V or cascaded supplies. As the output to the UUT needs to be isolated from earth ground (think isolated power supply), DC ground is isolated from earth ground. My questions:

1. For safety reasons, It is my intuition that the aluminum enclosure should be earth grounded. Is that correct?
2. In IEC Terminology, I believe the bulk of my board (12V and onwards) is considered an unearthed SELV circuit. I am maintaining reinforced isolation per IEC-60950. Assuming the enclosure is earth grounded, then I must also maintain this spacing with respect to any SELV component and the enclosure, correct?
3. In interest of a small form factor, I have clearances between components and the enclosure of less than the 4.0mm reinforced spacing required by the IEC standard. What would convention suggest here -- is robust dielectric tape sufficient to satisfy insulation requirements?

Thanks so much!

E

 

[Bluejets]

Most importantly.........Depends on the power supply unit.
As long as it is an isolated type supply unit then no need for any earth as your case encloses only 12v. (true??)

If the circuitry requires shielding in any way then the case should be connected to the negative of the 12v supply.

Do not get confused between earthing and ground in the high and low voltage systems.

Probably best to show some photos of what you have.

No idea what a UUT is...................

 

[pl510]

Most importantly.........Depends on the power supply unit.
As long as it is an isolated type supply unit then no need for any earth as your case encloses only 12v. (true??)

If the circuitry requires shielding in any way then the case should be connected to the negative of the 12v supply.

Do not get confused between earthing and ground in the high and low voltage systems.

Yes it is an isolated type supply. From a clearance/creepage and shielding standpoint, I would like to have the enclosure grounded to the 12V supply. However, in probing around the off-the-shelf power supplies I have in my lab, I noticed that the enclosures are all earth grounded and isolated from the output terminals. This would follow my initial assumption that the enclosure should be earth grounded and any isolated terminals to follow creepage/clearance requirements for reinforced insulation. Am I missing something here?

 

[AnalogKid]

No, you're doing quite sell.
1. yes.
2. only basic insulation between the secondary circuits and chassis earth gnd.
3. Kapton tape is the standard for this.

Most power supplies have the secondary circuit return separated from the chassis earth ground because they are designed for a general marketplace, where some will want that connection and some will not. It is far harder some people to break the connection without violating the certifications than it is for others to make the connection with a simple piece of wire. If we're talking aobut oem open frame or shoebox supplies, there almost always is a designated chassis ground connection point, such as a pc board mounting screw near the output connector.

Whether or not you connect the two is entirely up to you. In some environments, the earth connection brings noise into the secondary circuits. In others, it increases shielding effectiveness. There is no hard and fast rule.

ak

 

[Minder]

1. For safety reasons, It is my intuition that the aluminum enclosure should be earth grounded. Is that correct?

Thanks so much!

E

If your enclosure is earth grounded it is optional as to what is used for the low voltage insulation from the chassis.
M.