INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Jobs

Protection of ELV circuits

Protection of ELV circuits

(OP)
Hi Everyone

I am new to the forums and would like some advice for circuit design. I have an AC/DC (240VAC/24VDC) power supply unit supplying one DC circuit. My question is, should I have a circuit breaker on the DC side to protect the DC circuit? And if a circuit breaker is required on the DC side, in what way is it different to a laptop charger, which does not have protection on the DC side? I am interested in best practice from an Australian standards point of view.

RE: Protection of ELV circuits

"...a laptop charger, which does not have protection on the DC side..."

The above is probably a false assumption. Any laptop charger (perhaps excluding the worst of the knock-offs) would have current limiting on the output side.

RE: Protection of ELV circuits

It really does depend on the power supply in question.
For instance, I use plenty of 240VAC/24VDC switchmode supplies in the machines that we build, and these all have built-in short-circuit and overload protection which operates far more quickly than a circuit breaker. If I were to attempt to protect a 40A switchmode supply with a 40A CB, it would be clear from testing that the CB would never get the chance to trip, beaten every time by the PSU electronics.
However, if I were to attempt to use a cheap-and-nasty PSU without any short-circuit or overload protection, then it would be necessary to provide a CB of fuse to provide that protection, or risk frying both PSU and wiring loom under fault conditions.

RE: Protection of ELV circuits

Yep, in the absence of extra requirements, a switch/breaker on the 240V side and a fuse on the DC side would seem reasonable to me (in Aus land). The only reason to add a switch to the DC side is if the cord is long and integrated with the device it is powering. In that case it is convenient to have local on/off control.

RE: Protection of ELV circuits

Just echoing the others here -- your power supply probably already has protection on the low voltage side. Investigate it -- you should find a fuse, or, in the case of a smart switching supply, it's handled internally. Your power supply's data sheet should clue you in.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

RE: Protection of ELV circuits

(OP)
Thanks all, the datasheet does state that the PSU has overload (constant current limiting) protection that limits the PSU to 150% of the rated output power. The units rated power is 75W and rated current at 24V is 3.2A. This means that in short circuit condition, the current is limited to 150% * 3.2A = 4.8A @24VDC. However, the breaker on the 240VAC side is 2A, should I size it to be smaller so that if there is a fault on the DC side, the breaker on the AC side should trip as well? 4.8A@24VDC = 115.2W => 0.48A @ 240VAC

The spec for the PSU:
http://www.meanwell.com/search/DR-75/DR-75-spec.pd...

RE: Protection of ELV circuits

Quote (faraway73)

However, the breaker on the 240VAC side is 2A, should I size it to be smaller so that if there is a fault on the DC side, the breaker on the AC side should trip as well?

Probably not - when the PSU is first energised there is probably an inrush current to charge the capacitors. The 2A fuse might be sized to accommodate this so anything less might lead to nuisance tripping.

RE: Protection of ELV circuits

Just remember to size your DC wiring large enough to handle the fault (short circuit) current. If you want to go to smaller wiring, you'll need a fuse on the DC side, and then you can use wire large enough to handle the fuse current.

It is a bit trickier on the AC side, at least here in the USA, as UL says you have to use 18 AWG or larger on the AC side, even after fuses. UL trumps logic around here.

Z

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close