Can an ac and a dc circuit share a ground point?

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jaggedben said:
Most likely any of these AC powered supplies meet the rectifier exception. However, if you have a supply that does not meet that exception (e.g. battery, PV panel, dynamo) then yes to all of that, except that all the bonded stuff needs to be tied to a grounding electrode, not an AC EGC. If there are any AC supplies in the structure that are also required to be grounded then they'll all have to be tied to the same grounding electrode system.
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well, i need to dig more, fund plenty of documentation showing std bridge rectifier w/ DC common ground tied back to the service earth electrode (not to the EGC) where the N side of service is also ties to same earth ground. so in essence N and dc gnd are bonded together, but dc gnd is not tied to BC egc, etc. its an interesting topic.

dmxtothemax said:
Of course they can !
Look at a modern power amplifier
Lots of dc and ac grounds
all working together at the same time !
:cool:
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maybe. many devices that have sensitive items on the DC side rely heavily on isolation and want nothing to do with connecting anything from the DC side back to the AC side, etc.
 
FionaZuppa said:
please clarify for me.

i have a box, its a "DC power supply", the input is 1ph 120vac 60hz, the output id 75vdc.

when you say the DC has to be a DC grounded per NEC, are you talking about the input side of the box ?? you do not need the AC EGC to provide fault protection on the DC side of the box.

i have a box (dc power supply), input is 120vac 60hz 3-wire with proper egc. the DC is 75vdc 2 wire. the DC is electrically isolated from the AC via xfrmer. ok, so now two big wires run from the box down a raceway into a cabinet full of telecomm gear. are you saying that the telecomm box and gear needs to be bonded to each other and to DC common ground (typically the neg terminal), and then that tied to the power supply's 120vac EGC?
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The code says that where the DC voltage exceeds 60 volts one of the DC conductors must be a "grounded" conductor. That requires a GEC and a connection to a grounding electrode.

As I recall, the phone utilities ground the positive side of the DC system.
 
160805-2128 EDT

One example where it is impossible to connect a DC output lead to AC common, neutral, and/or EGC.

Place a bridge rectifier directly across a line to neutral source. This is the common input circuit in a switching power supply. If you now connect either + or - from the bridge output to common, then you will destroy the rectifier and probably trip the breaker in the AC supply. During a portion of a half cycle a diode short is applied between hot and neutral and the diode will be burned out and a very larger current will flow. The magnitude of the destruction is a function of the energy source and the size of the diode.

.
 
gar said:
160805-2128 EDT

One example where it is impossible to connect a DC output lead to AC common, neutral, and/or EGC.

Place a bridge rectifier directly across a line to neutral source. This is the common input circuit in a switching power supply. If you now connect either + or - from the bridge output to common, then you will destroy the rectifier and probably trip the breaker in the AC supply. During a portion of a half cycle a diode short is applied between hot and neutral and the diode will be burned out and a very larger current will flow. The magnitude of the destruction is a function of the energy source and the size of the diode.

.
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No argument with that. I have a notion that the question may have been about PV systems.
 
ggunn said:
How so? There are plenty of 1000VDC two wire ungrounded PV systems out there.
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Perhaps this explains things. I don't know much about PV systems, so i have no idea why they would make such an exception.

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).
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V. Grounding
690.41 System Grounding. For a photovoltaic power
source, one conductor of a 2-wire system with a photovoltaic
system voltage over 50 volts and the reference
(center tap) conductor of a bipolar system shall be solidly
grounded or shall use other methods that accomplish
equivalent system protection in accordance with 250.4(A)
and that utilize equipment listed and identified for the use.
Exception: Systems complying with 690.35.
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160806-1511 EDT

Find out inverters work, and this may tell you the answer. I have not studied them.

A 240 V output inverter with no output transformer almost certainly would not allow grounding of either +/- on the DC side.

.
 
Non-isolated inverters are more efficient but you can't ground one of the DC conductors or you have a direct short to AC neutral. (It's broadly similar to the rectifier situation that gar described a few posts back). Higher standards for ground-fault detection were developed for inverters with this architecture.

The 2017 NEC will introduce a new concept of 'functionally grounded' PV systems, which will effectively apply to all approved methods of ground-fault detection for PV systems that have some reference to ground but aren't solidly grounded. Which is pretty much all PV systems nowadays.

Chapter 6 modifies Chapter 2, as far as that question goes.
 
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post #48. you can "ground" to earth everything on the DC side, just dont tie anything on DC side to the AC side, etc. i see no safety issue with this method.
 
K8MHZ said:
And just ignore the NEC requirement for all grounding electrodes to be bonded together?
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i not sure you see what i mean to keep DC system separated from AC system

if you have a PV system then most likely you have an inverter, which may or may not be tied to the AC system. if its tied to poco AC then

Conclusions:
1. The drive toward ungrounded systems comes from trying to get more efficiency out of the inverters, specifically through the elimination of the isolation transformer. Without an isolation transformer the DC system must be ungrounded.
2. NEC permits the use of ungrounded systems providing equipment is adequately protected and that double-jacketed listed “PV Wire” is used.
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FionaZuppa said:
i not sure you see what i mean to keep DC system separated from AC system

if you have a PV system then most likely you have an inverter, which may or may not be tied to the AC system. if its tied to poco AC then
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That is not what I was asking about. It was this:

FionaZuppa
post #48. you can "ground" to earth everything on the DC side, just dont tie anything on DC side to the AC side, etc. i see no safety issue with this method.​
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Please start by explaining the above. A drawing would be most helpful. As stated, the ground electrodes for the DC side would not be bonded to the electrodes from the AC side, which seems to violate the NEC requirement of having all the electrodes bonded together.
 
FionaZuppa said:
post #48. you can "ground" to earth everything on the DC side, just dont tie anything on DC side to the AC side, etc. i see no safety issue with this method.
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It's really unclear if you're talking about system grounding or equipment grounding. If the first, no you can't do that and have a functioning non-isolated inverter. If you have an isolated inverter it's still not code compliant. If talking about equipment grounding, then what iwire said.
 
start at page 77 (i dont understand how they will take a rectified NEG back to AC neutral.)
https://ewh.ieee.org/r3/nashville/events/2011/Grounding-2011 color.pdf

this DC grounding talk is about taking one of the DC CCC's and tying it back to AC EGC. to me this just sounds bad. if the chassis and frames of the DC system are isolated from DC CCC's then surely bonding those chassis & frames items to AC EGC is "ok", but can still bring noise from the AC side right into the DC gear, which may not be an issue for a DC system that is doing crude power generation, i guess it depends on the inverters being used.

what would a PV/batt system have to do with AC is there is no inverter that would bridge the AC system to the DC system??

a full standalone DC system (PV+batts, or just PV) that uses inverter(s) for full "off-grid" application, i not seeing why you would tie anything from this power derived system to any other AC derived system.

for example sake, if i keep DC and AC isolated what is the hazard? if i carry AC EGC all the way to inverter chassis but keep everything on DC side isolated. the DC side has all frames/chassis bonding down to a earth rod, or, if the DC system uses chassis/frame NEG common ground, then DC NEG and all chassis/frame get tied to earth ground (not the AC earth ground / egc). you can still easily create a 3-wire DC system for catching fault the same way we do for AC.
 
FionaZuppa said:
start at page 77 (i dont understand how they will take a rectified NEG back to AC neutral.)
https://ewh.ieee.org/r3/nashville/events/2011/Grounding-2011%20color.pdf
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To be clear, all those diagrams have transformers in them so not relevant to non-isolated PV inverters, which is what I was talking about in post #48. Without the transformer you could not bond the rectified negative conductor to AC neutral. With the transformer it doesn't cause a short. That''s the same in a PV generator or a AC powered source.

this DC grounding talk is about taking one of the DC CCC's and tying it back to AC EGC. to me this just sounds bad. if the chassis and frames of the DC system are isolated from DC CCC's then surely bonding those chassis & frames items to AC EGC is "ok", but can still bring noise from the AC side right into the DC gear, which may not be an issue for a DC system that is doing crude power generation, i guess it depends on the inverters being used.
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True, for PV power generation noise on the DC side doesn't matter that much.

what would a PV/batt system have to do with AC is there is no inverter that would bridge the AC system to the DC system??

a full standalone DC system (PV+batts, or just PV) that uses inverter(s) for full "off-grid" application, i not seeing why you would tie anything from this power derived system to any other AC derived system.

for example sake, if i keep DC and AC isolated what is the hazard? if i carry AC EGC all the way to inverter chassis but keep everything on DC side isolated. the DC side has all frames/chassis bonding down to a earth rod, or, if the DC system uses chassis/frame NEG common ground, then DC NEG and all chassis/frame get tied to earth ground (not the AC earth ground / egc). you can still easily create a 3-wire DC system for catching fault the same way we do for AC.
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Could be lightning side flash, for one, if you have multiple grounding electrodes that are bonded by some inadequate communication wire or EGC. Ask why the NEC generally requires electrodes at the same building to be bonded to each other.
 
Ok, I will bite. Im confused :? IS this really a debate if an AC and DC system can be grounded in the same building? Or planet for that matter? :blink:
 
mbrooke said:
Ok, I will bite. Im confused :? IS this really a debate if an AC and DC system can be grounded in the same building? Or planet for that matter? :blink:
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You got me.:)

I am not sure if that is the question or even what types of DC supplies we are talking about. PV systems? Computer power supplies? Machine power supplies? DC drives?
 
iwire said:
You got me.:)

I am not sure if that is the question or even what types of DC supplies we are talking about. PV systems? Computer power supplies? Machine power supplies? DC drives?
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the OP didnt have clarity. it quickly turned into "yes" and "no" depending on what DC system it was, and pros & cons depending on types of equip used on the DC side, more precisely, the gear that connects between DC and AC system, like an inverter.

the crude answer was yes. the more defined answer depends on many factors.
 
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