28VDC Power Distribution Question

[faresos]

I’m working on a project for the National Guard where a 28VDC power distribution will be utilized for the Avionic testing equipment. I will be specifying a power converter 480V/3-Phase Input- 600A continuous 28VDC output. The negative terminal will be grounded. I have looked at existing installation and it looks like they have225A, MLO, 125/250VDC I-line panelboard with 20A/1P circuit breaker. I’m kind of confuse because they are using a single pole breaker (20A/1P) and I would thought they will use two poles breaker 20A/2P (positive and negative), However, some have indicated that a 2 poles breaker have been used as well for this application in different project. I assume the reading is always 28VDC between the positive and negative terminal because the negative terminal is grounded. What is the correct circuit breaker to be used 20A/1P or 20A/2P for the 28VDC power distribution? Is there any advantage from one to the another?

Thanks,

 

[adamscb]

I guess I'm slightly confused, how can you get a 28V DC reading off of a 125/250V DC panel, unless the panel is being utilized at a different voltage level than its nameplate? I'm not too experienced in the DC world, but I'll take a guess and say that single-pole breakers give you 125V, and two-pole breakers give you 250V?

EDIT - I think I see where you're coming from, in that this DC panel is entirely empty, or already being used for 28v loads. From my interpretation of what you wrote I thought they were already using that panel for 125/250v loads.

 

[petersonra]

I’m working on a project for the National Guard where a 28VDC power distribution will be utilized for the Avionic testing equipment. I will be specifying a power converter 480V/3-Phase Input- 600A continuous 28VDC output. The negative terminal will be grounded. I have looked at existing installation and it looks like they have225A, MLO, 125/250VDC I-line panelboard with 20A/1P circuit breaker. I’m kind of confuse because they are using a single pole breaker (20A/1P) and I would thought they will use two poles breaker 20A/2P (positive and negative), However, some have indicated that a 2 poles breaker have been used as well for this application in different project. I assume the reading is always 28VDC between the positive and negative terminal because the negative terminal is grounded. What is the correct circuit breaker to be used 20A/1P or 20A/2P for the 28VDC power distribution? Is there any advantage from one to the another?

Thanks,

Circuit breakers typically only need to open ungrounded conductors. So 1 pole breakers would seem to be appropriate to a grounded DC system. 240.15

 

[petersonra]

I guess I'm slightly confused, how can you get a 28V DC reading off of a 125/250V DC panel, unless the panel is being utilized at a different voltage level than its nameplate? I'm not too experienced in the DC world, but I'll take a guess and say that single-pole breakers give you 125V, and two-pole breakers give you 250V?

the square D breakers are also DC rated. I think for 48 V. They are also rated for 120/240VAC (or in UL parlelence, 125/250). I think the OP may have made a typo. Or not.

 

[Ingenieur]

sounds like some systems are grounded, some ungrounded/floating

do you have a data sheet for the ac/dc convertor?
solid state or a rectifier?

I would think they would want floating/ungrounded for testing

 

[Besoeker]

I’m working on a project for the National Guard where a 28VDC power distribution will be utilized for the Avionic testing equipment. I will be specifying a power converter 480V/3-Phase Input- 600A continuous 28VDC output. The negative terminal will be grounded. I have looked at existing installation and it looks like they have225A, MLO, 125/250VDC I-line panelboard with 20A/1P circuit breaker. I’m kind of confuse because they are using a single pole breaker (20A/1P) and I would thought they will use two poles breaker 20A/2P (positive and negative), However, some have indicated that a 2 poles breaker have been used as well for this application in different project. I assume the reading is always 28VDC between the positive and negative terminal because the negative terminal is grounded. What is the correct circuit breaker to be used 20A/1P or 20A/2P for the 28VDC power distribution? Is there any advantage from one to the another?

Thanks,

If the negative terminal is grounded would you really want your CB to open that?

 

[faresos]

The panel is utilized at different voltage. The existing panel name plate is 225A, 125/250 VDC.

The converter is Unitron : UltraLite FlatPak Series. Ouput Voltage per the data sheet : 28 VDC, 2-wire, Grounded Negative

 

[Ingenieur]

single pole will do on a grounded system

could not find an install guide
do they still want 3 wires run to the point of use? +, - and gnd?

 

[faresos]

single pole will do on a grounded system

so the single pole breaker needs to be bolted at the positive terminal only and the negative terminal no breakers will bolt on it, correct?

the reason I'm asking this because the existing installation has breakers at each side of the panel (you have 20A/1P breakers at circuits 1, 3, 5, 7 & 2, 4, 6, 8. Does that mean all these breakers bolt to the positive terminal?

forgive my ignorance as I'm not much familiar this system.

thanks all for all your help!

 

[Ingenieur]

so the single pole breaker needs to be bolted at the positive terminal only and the negative terminal no breakers will bolt on it, correct?

the reason I'm asking this because the existing installation has breakers at each side of the panel (you have 20A/1P breakers at circuits 1, 3, 5, 7 & 2, 4, 6, 8. Does that mean all these breakers bolt to the positive terminal?

forgive my ignorance as I'm not much familiar this system.

thanks all for all your help!

yes
tied the 2 buses together, it seems
are all negatives tied to the neutral bar?

or is the pos and neg each wired to a bus?
pictures? always take pictures if not

it's better to ask questions up front
NOT asking would be ignorant lol

for other members
is a 120 single line panel made?

 

[Besoeker]

sounds like some systems are grounded, some ungrounded/floating

do you have a data sheet for the ac/dc convertor?
solid state or a rectifier?

I would think they would want floating/ungrounded for testing

At 600A I would imagine it is a rectifier. I haven't checked but I think it is doable with isolated encapsulated modules

 

[adamscb]

To be honest I'm not sure if these are even made, but how about buying a 480v three phase to 28/56VDC converter? That way you could utilize all breaker spots in the panel?

 

[Ingenieur]

At 600A I would imagine it is a rectifier. I haven't checked but I think it is doable with isolated encapsulated modules

I looked at the data sheet, it appears solid state

http://www.unitronlp.com/includes/PDF/UltraLite FlatPak_P_RevG.pdf

 

[GoldDigger]

To be honest I'm not sure if these are even made, but how about buying a 480v three phase to 28/56VDC converter? That way you could utilize all breaker spots in the panel?

For the panel on the DC output side the phasing on the input side does not really matter. And you can tie all of the buses together on the AC panel used for DC since you will never be trying to share neutrals.
No such thing as an MWBC for DC unless you have both positive and negative polarity outputs from the DC supply.
And in that case you would want to use a single phase three wire panel, not a three phase panel.

 

[adamscb]

For the panel on the DC output side the phasing on the input side does not really matter. And you can tie all of the buses together on the AC panel used for DC since you will never be trying to share neutrals.
No such thing as an MWBC for DC unless you have both positive and negative polarity outputs from the DC supply.
And in that case you would want to use a single phase three wire panel, not a three phase panel.

Sorry I didn't specify, what I meant by 28/56VDC is one +28VDC supply and one -28VDC supply. Could you not come off of this dual voltage power supply and feed one leg in the DC panel with +28, the other with -28? That way you could use all of the breaker spots in the panel. From the way I understood the OP, the existing DC panel is suitable for this application. And I'm slightly confused - from the OP he mentions a 125/250VDC panel being used, but not an AC panel that is being used for DC, or am I just extremely confused here?

 

[GoldDigger]

Sorry I didn't specify, what I meant by 28/56VDC is one +28VDC supply and one -28VDC supply. Could you not come off of this dual voltage power supply and feed one leg in the DC panel with +28, the other with -28? That way you could use all of the breaker spots in the panel. From the way I understood the OP, the existing DC panel is suitable for this application. And I'm slightly confused - from the OP he mentions a 125/250VDC panel being used, but not an AC panel that is being used for DC, or am I just extremely confused here?

Yes. But so am I.
I am pretty sure that the OP is talking about repurposing an AC panel to handle the DC output(s) with breakers for several circuits coming from a larger common supply.

 

[Jraef]

Quit thinking of the panel as anything other than a convenient way of getting bus bar and breakers that plug into it. If it is a grounded DC system, connect [+] to both sets of bus and you can use them all, then connect the [-] circuits to the neutral bar (again, forget the term "neutral" as it would relate to AC, it's just the term for the bar itself). If it were NOT a grounded [-] DC system, then you would use 2 pole breakers and put [+] on one bus, [-] on the other one.

Also, sometimes people use 2 pole stand alone breakers on DC systems when the DC voltage is higher than the single pole DC rating of the breakers. So for example if you had breakers rated at 30VDC and you have a 48VDC system, you can often get 48VDC by wiring 2 poles in series with each other, because the arc clearing distance required for DC is higher, but is cumulative between contacts so more contacts in series results in being able to switch higher DC voltages. But that would not work on a panelboard system, it only works on stand-alone breakers. I'm just bringing this up to explain a possible reason behind why you have seen some DC systems use 2 pole breakers when it is grounded [-].

 

[elec_eng]

Quit thinking of the panel as anything other than a convenient way of getting bus bar and breakers that plug into it. If it is a grounded DC system, connect [+] to both sets of bus and you can use them all, then connect the [-] circuits to the neutral bar (again, forget the term "neutral" as it would relate to AC, it's just the term for the bar itself). If it were NOT a grounded [-] DC system, then you would use 2 pole breakers and put [+] on one bus, [-] on the other one.[/I]

Just curious, what would be the reasons to choose between "Grounded DC system" vs "Non-Grounded DC system"? Pro's and Con's?