NGR question

[mbrooke]

assume a machine with 3 ph wye (or delta w/zig-zag derived neut) plus egc
a phase faults to the frame of the machine
a fault i returns to the X0
with no ngr only limitation is conductor and xfmr Z
frame rises to ph-n v (bad for guy touching machine)
all vdrop is across the egc, vn on the frame dropping to 0 at the X0

Now interpose an ngr in the ngr-X0 path
Fault current is limited by the ngr
so now the v drop is divided across the egc and ngr and fault i << less than no ngr
if vph is 480 then vn 277
total loop = ngr + egc = 18.5 + 2.5 (for example) = 21 Ohm
i fault = 277/21 = 13.2 A
across ngr 18.5 x 13.2 = 244
on frame 277 - 244 = 33 dropped along egc up to ngr(simple kvl loop)
Much smaller shock potential
all done for personnel protection
these faults are common in mining since cables (no conduit) is used

So this is in case the person comes in contact directly with a phase? Make sense, basically the NGR limits the current through the person.

 

[topgone]

So this is in case the person comes in contact directly with a phase? Make sense, basically the NGR limits the current through the person.

NGR limits the possible fault current, hence mitigate the damage it brings, that is.

 

[GoldDigger]

NGR limits the possible fault current, hence mitigate the damage it brings, that is.

And it is very unlikely to be large enough compared to the resistance of a person to have any effect on the current through him.

Sent from my XT1585 using Tapatalk

 

[Sahib]

So this is in case the person comes in contact directly with a phase? Make sense, basically the NGR limits the current through the person.

Yes but if the person comes in contact directly with the neutral of NGR system, while there is a phase to ground fault, he is likely to be electrocuted because he would be in touch with almost full phase voltage.

 

[mbrooke]

Yes but if the person comes in contact directly with the neutral of NGR system, while there is a phase to ground fault, he is likely to be electrocuted because he would be in touch with almost full phase voltage.

Wont it be 277? Sorry if my thinking is off tonight, I'm out there right now :lol:


But yahhh, I like Ingenrieers setup. Never thought of NGRs in that light.

 

[Ingenieur]

And it is very unlikely to be large enough compared to the resistance of a person to have any effect on the current through him.

Sent from my XT1585 using Tapatalk

if no ngr frame v 277
i thru 1000 Ohm person 277 mA
must clear 0.4 sec

if ngr frame v 50 v
i thru person 50 mA
must clear 12 sec

 

[topgone]

if no ngr frame v 277
i thru 1000 Ohm person 277 mA
must clear 0.4 sec

if ngr frame v 50 v
i thru person 50 mA
must clear 12 sec

The fault current will have two paths once a person touches the ngr frame. One goes thru the ngr and the other passes thru the person.

 

[Ingenieur]

The fault current will have two paths once a person touches the ngr frame. One goes thru the ngr and the other passes thru the person.

Of course
but if frame is 277
man sees 277/1000 or 277 mA
ngr 277/20 = 13.85 A

works out the same
eff R 20||1000 = 19.6 Ohm
total i 277/19.6 = 14.13 A
Current divider
person 20/1020 x 14.13 = 0.277 A or 277 mA
ngr 1000/1020 x 14.13 = 13.85 A (14.13 - 0.277)

we HAVE TO HAVE the parallel path
or i fault thru ngr is only 277 mA and never trips the cb (5-7 A setting)...fatal in <1/2 sec
that is why we monitor the egc, if lost/open cb trips and can't be reset till fixed

 

[mbrooke]

Of course
but if frame is 277
man sees 277/1000 or 277 mA
ngr 277/20 = 13.85 A

works out the same
eff R 20||1000 = 19.6 Ohm
total i 277/19.6 = 14.13 A
Current divider
person 20/1020 x 14.13 = 0.277 A or 277 mA
ngr 1000/1020 x 14.13 = 13.85 A (14.13 - 0.277)

we HAVE TO HAVE the parallel path
or i fault thru ngr is only 277 mA and never trips the cb (5-7 A setting)...fatal in <1/2 sec
that is why we monitor the egc, if lost/open cb trips and can't be reset till fixed

How do you detect an open EGC?

 

[Ingenieur]

How do you detect an open EGC?

2 main ways
a relay puts out a very low v/i signal on a pilot wire
the pilot is tied to the machine frame
the signal loop rtn is the egc
if the relay does not detect the rtn signal on the egc it trips
it also calibrated so >3 Ohm or so attenuates the signal enough to drop the relay out

the other puts a tone (400 Hz) via a tuned hi Z filter (hi pass) onto the phases
at the machine another filter extracts it from the phases and puts it on the egc
a ct with the relay detects it on the egc, if not trips
there are measures (chokes, diodes, etc) to make sure tones from adjacent ckts can't 'fool' it via parallel gnd paths

 

[mbrooke]

2 main ways
a relay puts out a very low v/i signal on a pilot wire
the pilot is tied to the machine frame
the signal loop rtn is the egc
if the relay does not detect the rtn signal on the egc it trips
it also calibrated so >3 Ohm or so attenuates the signal enough to drop the relay out

the other puts a tone (400 Hz) via a tuned hi Z filter (hi pass) onto the phases
at the machine another filter extracts it from the phases and puts it on the egc
a ct with the relay detects it on the egc, if not trips
there are measures (chokes, diodes, etc) to make sure tones from adjacent ckts can't 'fool' it via parallel gnd paths

I want your JOB! :happyyes::happyyes::happyyes: Do you have a link to the system btw?