Safety grounds - several questions

[BrianE22]

We're putting together a system to test our product. We have a 24 VDC isolated power supply that controls a motor. A hand held, plastic pendant switch uses the 24 volts to turn a motor in either direction. I assume that the low side of the 24 VDC control system should be attached to an earth ground for safety. The question is, how robust (current carrying ability) should the earth (and therefore, power supply low) connections and wiring be?

The 24 VDC control only provides 15 mA so we're using small wires and terminal blocks. But that would seem to be inconsistent with the need to provide a robust path to ground in the case of a fault. So how can you get by with low current wiring and terminal blocks and still satisfy the grounding needs?

 

[Skogsgurra]

In EU, there is the SELV (Safety Extra Low Voltage) with an upper level being 50 V. Anything below that level can be used without PE or other safety measures - you need to have fuses or similar overcurrent protection, of course.

Not fluent in US code. But can imagine there are similar rules.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[BrianE22]

Thanks for the reply Gunner. Excuse my lack of knowledge, what does PE stand for?

 

[Skogsgurra]

PE=Protective Earth

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Noway2]

SELV applies in the US also. I am thinking, but not 100% certain, that the limit is a little lower, somewhere in the 40s but near 50V. 24Vdc is well below the limit and does not require a safety ground.

 

[MacGyverS2000]

Noway,

The value of 48V sticks in my head, but I make no guarantees that's the reason, so I never posted...

Dan - Owner

http://www.Hi-TecDesigns.com

 

[mcgyvr]

Typically SELV refers to anything not exceeding 42.4V peak or 60VDC.

 

[peebee]

NEC 250.162 requires DC systems from 50-300v to be grounded. To the best of my knowledge, your 24VDC system is not required to be grounded in the US. Any metallic non-current-carrying metal parts (conduit, etc) would need to be grounded per NEC 250.169; it's unclear if you have any such metallic objects in your system or not. The required conductor size is covered in NEC 250.166.

 

[Warpspeed]

Similar in Australia.
As long as the extra low voltage mains isolating transformer is of an approved type, with a split insulated bobbin, and adequate approved insulation, the secondary circuit need not be grounded.