Like I said, you may get away with it. That doesn't mean it is either best or safest practice.
As iWire has noted, it is a minimum standard.
My priority would be focussed on safety. Wouldn't yours?
Consequences of no Neutral Conductor
- Thread starter soldier_sss
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As iWire has noted, it is a minimum standard.
My priority would be focussed on safety. Wouldn't yours?
That was more conventional when all the loads were delta connected motor and transformer windings. Grounding detection, alarms, and signage are/were required.
Today's loads are frequently VFD's. The VFD front end has a filter and transient suppressor network that is internally Y connected and grounded. So, in the ungrounded 3p 3w system, regardless of if the upstream transformer secondary is Y and only loaded delta, the first ground fault on the load side is already present, in the drive at the grounding point of its internal Y connected filter network.
So a second ground fault will be either line to line or line to ground, and the fault current path is through the first grounding connection, actually through the first ground fault in series, which is inside the VDF, all of them. The fault path is through the VFD's Y filter network. The drive should be expected to go poof.
The some drives are rated to operated on ungrounded Y or delta sources and those instructions specifically say to lift or remove the drive's filter network internal bonding screw. In the average case, the plant will not even know it's power system is ungrounded, and certainly not know that if it installs a drive it has to remove the drives internal grounding bonding screw. It is an existential hazard.
Ungrounded power supply system is one of the safest as discussed in this article:
http://www.bender-ca.com/fileadmin/bca/doc/Ungrounded_systems.pdf
Bender would infer that, wouldn't they.
But read the article carefully. All of it.
Read. What is the point?
OK. Explain, in your own words, why it is one of the safest systems.
If an individual touches a ph and frame on an ungrounded system no shock
or very small depending on C coupling
same applies if ph to frame and you touch frame
A shock results on a grounded system
this is mitigated by the egc <<< than body Z
and if it happens with no contact cb trips
even with touching it clears and the current the body sees is in parallel with the egc
all this depends on the integrity of the gnd system and fault Z
- Location
- Massachusetts
My understanding is that the 'C coupling' in any industrial building is more than enough to knock you on your butt.
A better example might be a isolated power system as used in am operating room but I highly doubt Bes needs any example.
Depends
voltage
installation methods, cable, individual cond, etc
laid on earth
etc
could kill you
perhaps not even felt
you don't see the whole C current
it is limited by body Z ~ 1000 ohm +/-
which is still more than enough
if you have 2 A on a 480 system Xc 240 ohm
so with a body is 480/(240 + 1000) = 400 mA
all just for illustration
the big issue is the fault is not sensed/interrupted so the person is screwed
in most cases cap coupling is fractions of an amp so If is much less
- Location
- Illinois
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- retired electrician
If the system can source "fractions" of an amp to ground, it can be a serious shock hazard.
I had a old solenoid voltage tester that took about 20 mA to pull in the coil.....almost all of the ungrounded industrial systems that I checked could pull in the tester. A system that can supply 20 mA between a circuit conductor and earth is a system that can be a shock hazard.
If the phase is grounded the V goes to 0
no potential across his body (different if the body is the gnd path)
if a charging I flows it is apportioned between the fault and the 1000 ohm person
Assume a current of 2 A, very high assumption
so if the fault is 10 ohm the guy sees 10/1000 x 2000 = 20 mA
there are varying values for let go current and this is close
the current a person can survive is approximated by
ma = 165/(sqrt time) where the 165 is body wt
time = (165/ma)^2
so 165 ma for 1 sec
delta systems are inherently safe if provided with protection, bonded and properly maintained
when I measure 5 MVA 480 delta systems we typically see <1/2 A
same on a wye with 15A ngr which also includes insul leakage
At that level the person would see 10/1000 x 500 or 5 mA
and the fault is usually much less than 10 ohms, crushed cable and so on
both gnd and un-gnd are safe if engineered, installed, operated and maintained properly
- Location
- Massachusetts
Ungrounded systems are not installed for personal protection because they do not provide it. They are installed for continuity of service.
niether are grounded systems
both will kill you if mishandled or in the wrong place at the wrong time
shock hazard for a ground fault (person in series or parallel) is less with an ungrouded system
they are used not only for continuity of service but in applicatons where likelyhood of a gf is high
like portable cables
handling a wet cable with insul damage is safer
I've only seen 1 fatality on a delta ug
motor insul broke down on a phase
gf on another phase on a conveyor
guy touched both: fatality
no bonding
seen many on a gnded ngr set-ups
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- Location
- Illinois
- Occupation
- retired electrician
For a system of any size, it doesn't make any difference ... the shock hazard is almost identical. The current through the person is limited by the impedance of the person and the source being grounded or ungrounded does not make any real difference unless the ungrounded system is very small (like systems used in operating rooms).
On a delta with low charging current (as most are) you will see mA
Assume X 2000 or 1/4 A per ph ~ 3/4 total
1000 ohm + 2||Xcc (typically in the 1000's)
total path 2 cos 30 x 2000 + 1000 ~ 4500 ohm
gf i thru person ~ 480/4500 ~ 100 mA
most systems much lower
on a grounded system Vln/1000 ~ 277 mA
note niether system will trip
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- Location
- Massachusetts
Well thanks for assuming we are all dolts.
I will maintain that ungrounded systems are not installed to prevent shocks to personal.
Well thanks for assuming we are all dolts.
I will maintain that ungrounded systems are not installed to prevent shocks to personal.
no need for snarky
no industrial system is installed for personnel protection
it is installed to power equipment
but you try to minimize risk
industrial electrical accidents are 90% lock out/ tag out
Taking the UK systemas an example, normally the distribution is from a star secondary of a distribution transformer and 400/230V. The star point is connected to earth (ground). so, for the single phase residential user the shock hazard is 230V and thhat can be mitigated by an ELCB.
If the system is floating it could be any voltage to ground. If you had any one phase with a ground fault the hazard is then 400V. Even as a dumb Brit, I can't see how that could be construed as safer.
- Location
- Massachusetts
That was far from snarky considering you told us electricity could hurt us.
You jumped into this thread by saying ungrounded systems are safer, I disagree with that view.
Those facts do nothing to show ungrounded industrial systems are safer to the personal working around them.
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