1Pole or 3 Pole DC Circuit Breaker Application 4

[nightfox1925]

I would like to inquire if it is a Code requirement to use 2 pole circuit breaker for 125Vdc, 2 wire system. The switchgear used 2Pole breakers while the panelboard used 1pole circuit breakers...does this matter?

 

[davidbeach]

What is the DC rating of the circuit breaker(s) in question? Code requires that equipment be used per its ratings and installation instructions.

 

[nightfox1925]

As per data gathered, its 250Vdc breakers.

 

[stevenal]

NEC 240.15:
Overcurrent Device Required. A fuse or an overcurrent trip unit of a circuit breaker shall be connected in series with each ungrounded conductor.

 

[nightfox1925]

Thanks stevenal. The Charger specs indicates that the output can be converted to 2 wire, with grounded conductor. The earth fault monitor should be disabled. Then, the 1 pole distribution can be used.

 

[dpc]

Grounding a 125 V dc system may not be a good idea. What is the 125 V dc used for?

 

[nightfox1925]

it is being used for switchgear DC control applications. For the cirucit breaker closing and tripping circuits.

 

[davidbeach]

Leave the 125VDC ungrounded, you will enjoy life more.

 

[nightfox1925]

May I know for my own knowledge as to why grounding it would not be a good idea? We are planning to ground it to an isolated ground. Thank you.

 

[davidbeach]

A ground fault on a grounded DC control system will certainly blow a fuse or trip a breaker. No telling what won't work properly or what will operate improperly - a lot depends on where the ground fault happens.

A ground fault on an ungrounded DC control system will not operate any protection nor cause misoperation in a properly designed system (watch your control levels), and can be alarmed to allow one to go looking for the fault without having everything go to heck in the mean time.

 

[stevenal]

I agree with davidbeach. The point of the DC battery system is to have uninterrupted power for tripping. A grounded system ensures the first fault will make that power unavailable. Ungrounded requires the second contingency to get that result.

 

[dpc]

Grounding to an "isolated" ground doesn't really change anything. Ground is ground.

Dc control power supplies should definitely be ungrounded for improved reliability.

 

[nightfox1925]

I got it. And I already advised to convert the system to ungrounded and use 2P breakers.

Thank you very much

 

[PHovnanian]

It depends on what the 'earth fault monitor' mentioned by nightfox1925 does. And what the consequences of losing the DC supply are. If the system is supervised, then raising an alarm upon detecting the first ground fault might be a good idea. Or one can use the fault monitor to provide an orderly shutdown of the system prior to removing the DC control supply.

The other property of a grounded system (that one might want to avoid) is a ground fault that causes some unintentional system operation. Control circuitry is supposed to eliminate these by prohibiting switching of the grounded side of a relay coil, for example. But with complex systems, finding every possible sneak circuit can be difficult.

 

[slavag]

Hi.
Substation DC supply will be ungrounded only.
I agree with it 100%.
Storngly recommend DC control system with UV/OV+Ripple+Isolation monitoring.
Best Regards.
Slava

 

[nightfox1925]

I also advised to provide DC supervision relays on each DC circuit of each breaker cubicle.

 

[slavag]

Be carefull with this.
Actually strongly recommended only one DC monitoring per DC bus, in case of coupling of two busses start problem with isolation monitoring. I recommend ask a vendor, Bender for example.
Best Regards.
Slava

 

[jraef]

Although not applicable to this situation, I thought I would add this for posterity if someone is doing a future search on DC circuit protection for Solar PV systems.

In the US, Solar PV arrays that have circuit breakers or fused switches for protection are only allowed to break the positive pole, never the negative, and the negative must always be grounded.




"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[cranky108]

Has anyone else seen the phone company attempting to ground the positive of a battery set?
It did not work well with the UPS negative ground (SEL UPS has a negative ground).

 

[stevenal]

690.35 Ungrounded Photovoltaic Power Systems.
Photovoltaic power systems shall be permitted to operate with ungrounded photovoltaic source and output circuits where the system complies with 690.35(A) through (G).

I don't see where it is forbidden.