24V SELV supply - 2 pole breakers required?

[DrDrreeeaaa]

Hi All,

Have come across an unusual situation in our game (industrial control panels) where the engineer has specified the 24VDC to be SELV (floating).

Should 2 pole breakers be used for branch circuits?

I understand that 2 pole breakers are recommended for ungrounded systems - my understanding is that this is mainly for the case where if a breaker trips then both ungrounded conductors are isolated, as opposed to one side being isolated but the other side remaining live.

But do you think this is neccesary for SELV? I am talking about a floating 24V from a switch mode power supply.

The loads which are supplied are typical field devices, limit switch PLC inputs (PNP) and valve outputs.

Strugling with this one. Thanks all.

 

[GroovyGuy]

Howdy 123,
I usually prefer to ground one side of a 24vdc power system and therefore use 1-pole CBs. However, from time to time I do come across an (existing) ungrounded 24Vdc system, and approx. 50% of the time theses systems (incorrectly) use 1-pole CBs. I will then either;
1) Ground one side of the system, or
2) replace the 1-pole CBs with 2-pole CBs.
I much prefere a solidly grounded control system as it allows to to check potentials in the field (ie line-to-ground).
(ie With an ungrounded system, it is much more difficult to check potentials in the field as everything is floating).

For those who really want a floating system, why not use a grounding resistor on one (or both) sides of a 24Vdc system, At least with this type of system you can monitor the system (and detect when the system experiences it's first ground). I have never done this on a 24VDc system before, but did desgign & install such a sytem on a 250kW 300Vdc battery string. It worked rather well.
Comments?
GG


"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (1847-1931)

 

[John2025]

GG, for the same reason as you I always ground the negative.It seems like you'd still need a 2-pole if you ground through a resistor, but I'm curious to hear what the floating supply crew thinks about the noise issues with a resistance ground
John

 

[DrDrreeeaaa]

Thanks all

I'm coming across lots of Oil and Gas projects where the DC supply is floating.

GG, unfortunately I can't ground the supply as it is against the customer's requirements.

But regarding double pole breakers for a SELV supply, what does everyone think?

1) Am I correct in saying the purpose for a double pole breaker/fuse is to ensure both live conductors are isolated in the event of a fault on either line
2) However, since the supply is SELV, is this really neccesary? Protection is provided by the SELV supply and inadvertent contact with one of the lines wouldn't cause a shock in any case

Thanks

 

[John2025]

I didn't consider SELV and don't have any experience with it so never mind my comments.

 

[itsmoked]

Would someone please explain why a two pole breaker on the line side is needed when the output is totally galvanically isolated from the line side regardless of the SELV rating? They're essentially separately derived sources anyway, no?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Hi Keith,

The breaker is on the load side as far as I can see?

123MB - yes, you do need to protect both poles of a floating supply against overcurrent. The main reason for this is that if a first earth fault occurs on the positive pole of a circuit protected by a large breaker and then a second earth fault occurs on the negative pole of a much smaller circuit, the smaller circuit won't disconnect unless there's an overcurrent element in the negative leg. On large battery systems that can lead to some really spectacular events if protection isn't provided in both poles.

Unless this is a really big SMPS there's virtually no prospect of tripping an MCB on short circuit before the PSU goes into shutdown to protect itself. If you add a lot of capacitance or a decent-sized battery onto the DC rail then you might be able to trip the breaker, but with only 24V available you won't need much conductor length before the magnetic element won't operate even on a dead short. You might want to consider fuses instead of breakers with their much steeper time/ current curve?

 

[itsmoked]

Oh. I didn't get that. That make a lot more sense, thanks Scotty.

Keith Cress
kcress -

http://www.flaminsystems.com