Fuse in both lines of 24VDC

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According to 240.15(A), all ungrounded conductors are to have overcurrent protection. Does this mean that a 24VDC power supply (ungrounded output) should have both the 24VDC and the 0VDC lines fused?

This power supply is being used for instrumentation and controls.

I have not seen this in practice in industry, but I am being told this is what they want to be up to code.

Any thoughts?
 
According to 240.15(A), all ungrounded conductors are to have overcurrent protection. Does this mean that a 24VDC power supply (ungrounded output) should have both the 24VDC and the 0VDC lines fused?

This power supply is being used for instrumentation and controls.

I have not seen this in practice in industry, but I am being told this is what they want to be up to code.

Any thoughts?
If it is not grounded, it should be fused. (It only becomes 0DC if it is grounded.)
 

Smart $

Esteemed Member
Location
Ohio
According to 240.15(A), all ungrounded conductors are to have overcurrent protection. Does this mean that a 24VDC power supply (ungrounded output) should have both the 24VDC and the 0VDC lines fused?

This power supply is being used for instrumentation and controls.

I have not seen this in practice in industry, but I am being told this is what they want to be up to code.

Any thoughts?
If the power supply is Class 2 or 3, I don't believe there is any requirement to fuse conductors on the load side. Refer to Article 725.

However, I recommend fusing each instrument and control circuit (one side) just to prevent a short from shutting down the entire connected system.
 
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SG-1

Senior Member
I agree with weressl, if the DC positive or negative is not grounded, then both get fuses. I see this every day with control circuits. These control circuits are usually powered by station batteries.
 
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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
If the power supply is Class 2 or 3, I don't believe there is any requirement to fuse conductors on the load side. Refer to Article 725.

However, I recommend fusing each instrument and control circuit (one side) just to prevent a short from shutting down the entire connected system.
Agreed, for the most part.

I am kind of torn between wanting the whole thing to trip so it is detected and only having a small part of the system go down and not noticing it.
 

smoky_conductor

New member
Location
michigan
24VDC supply protection

24VDC supply protection

I do thank all of you, those that direct the site and those that ask/answer the questions. kudos...


my Dad told me many years ago that the whole idea behind a fused circuit was to keep the smoke in the small glass holder that was sealed at both ends. it took me a few years to fully appreciate what he was laughing about while he was explaining but when it finally caught up to me, and when I too was finished grinning I realized just what he was saying. boy do I miss the man, the best teacher I ever had...
 

blargh

Member
Location
Redwood City, CA
Occupation
Design Engineer
On a piece of equipment I was working on to bring up to UL compliance (UL 508A), which used several 24 VDC power supplies, the inspector required that we install a jumper between the AC input's EGC and the (-) terminal of the DC output. This will produce the "0 VDC" that you mention.
 
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