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Morbid Ignorance

4K views 15 replies 3 participants last post by  Meadow 
#1 ·
This is to funny :laughing: And scary :censored:

Apparently spending my whole life in the USA, I have no clue what Im talking about. :no: Supposedly we clear all ground faults via ground rod, hence why the 25 ohm rule exists in the NEC?

Normally Id turn a blind eye, but supposedly he is applying NFPA 70 work overseas and not using a bonding jumper. I think I now know why our troops are being electrocuted :censored:

See starting from post 11:

http://www.electriciansforums.co.uk...ks/96409-adding-test-procedures-nec-code.html
 
#3 ·
I have no idea what he means by TT earthing....

~C(blissfully posted)S~

Good point, I shall explain. :)

TT earthing (an IEC term) is a system where no main bonding jumper exists at the service. All faults are cleared via ground rod or plate. However, because getting low earth resistance is very difficult if not impossible GFCI (RCD) breakers are used on all circuits. As long as the ground rod can pass at least 30milliamps (most Euro GFCI breaker ratings) the system is safe.

However, if I did such a thing even with GFCI brekears it would violate the NEC. If I did it with our standard thermal magnetic breakers, a ground fault would never clear.
 
#11 ·
Thx, I find the engineering of other systems rather fascinating Meadow. Someone thought them up , and had them validated by peers

In the case of the TT (which at first sniff violates every grounding theory we've been trained to) can we take it as dependent on an RCD main ? I'm of the understanding that most RCD resi mains across the pond are 300ma.

As this trips on an imbalance over that level , much like our similar gfci's no egc is really needed. So the GEC's must be for lightning protection more than a return path.....? Yes it's an odd arrangement, no i do not know how prevalent it is over the pond?


One version i DO agree with is the PEN configuration. This incorporates ONE MBJ only. One can see any great distance as assuming VD , but the trade off is no multiple MBJ's throughout a structure. I'm also under the impression the PEN advocates consider our penchant for earthing neutrals in close proximity contra theory.

~CS~
 
#12 ·
Thx, I find the engineering of other systems rather fascinating Meadow. Someone thought them up , and had them validated by peers
Welcome :) I find it fascinating as well, it gets me to think about grounding, earthing and bonding in new ways. A lot of these systems evolved over decades of use; trial/error from my understanding. But a lot of it has been refined over the years through statistics and theoretical analysis, much like our codes and standards.


In the case of the TT (which at first sniff violates every grounding theory we've been trained to) can we take it as dependent on an RCD main ?
TT is new to any American EE or sparky. However, the only difference between TT and TN/our systems is a main bonding jumper. A ground bar still exists in the panel, all circuits have an EGC and water, UFER, plumbing, gas pipes ect are all bonded to the panel ground bar as well. The difference begins in that earth rods or plates are added which ensure conductivity to the soil.

I'm of the understanding that most RCD resi mains across the pond are 300ma.
They may be 50ma, 100ma, 500ma , even 1000ma (depending on a variety of factors like disconnect times, earth loop impedance, ect, ect). 3 schemes exist. One is having non RCD mains and having all circuits on individual 30ma RCD breakers. The 2nd option is standard branch circuit breakers but with an RCD main, 100 or 300 ma being typical for residential. The 3rd way is highly recommended by IEC sparkies: All branch circuit breakers are 30ma RCDs with a 100ma delayed trip RCD main. Should either RCD fail, one will back up the other. A delayed trip main is used to allow for selective coordination so a fault on a branch circuit with a functioning RCD will not pop the main.


As this trips on an imbalance over that level , much like our similar gfci's no egc is really needed. So the GEC's must be for lightning protection more than a return path.....? Yes it's an odd arrangement, no i do not know how prevalent it is over the pond?
And EGC is still needed from exposed metal parts to the panel ground bar. Equal potential bonding is still done within the structure and considered essential to safety. GECs serve a 2nd purpose beside lightning protection: they act as EGCs. The soil is used as conductor to clear a fault. Because the resistance is over 10 ohm 99% of the time, an RCD is used to clear the fault.


These systems are very common in some countries, for example rural UK. TT has an advantage that when no other metallic paths exist between the customer and other customers/transformer fault currents are extremely limited, behaving almost like a high resistance grounded system. No stray voltage or ground currents exist which can be a big help with sensitive electronics.

In some cases TT earthing is mandatory over TN-C. :eek: One such example are caravans (mobile homes/trailers to us). Here only TN-S or TT can be
used. If TN-C was used and the PEN broke, the metal frame would energize up to 230 volts between it and earth. This is actually why mobile homes in the US always required a 4 wire feed and required 4 wire stoves and dryers well before the 1996 NEC.


BTW, Im sure Mr. Chicken Steve is wondering: How was TT earthing accomplished before RCDs? Well, I have an answer for the.: VOELCBs

Starting at post #24, going into detail at 29:


http://forums.mikeholt.com/showthread.php?t=165856




One version i DO agree with is the PEN configuration. This incorporates ONE MBJ only. One can see any great distance as assuming VD , but the trade off is no multiple MBJ's throughout a structure. I'm also under the impression the PEN advocates consider our penchant for earthing neutrals in close proximity contra theory.

~CS~

PEN is the cheapest, and simplest, but has 2 down falls: one being open neutral the other constant current through the earth/metal parts. Personally IMO the best system is TN-S, but out of all the systems on earth it is the least used for services. The reason being cost. POCOs don't want to run an extra conductor, and would much rather let the customer pick an RCD or main bonding jumper.

Norway used IT for decades, but got it very wrong when they did not (failed to) interconnect all grounding systems between homes through a ground wire on the pole. When an IT system faults it turns into a TN, or in this case a TT system. When a ground fault occurs in another structure on an opposite phase 230 volts potential now exists between 2 structures. Both a serious fire and electrocution hazard. It is rumored to be a major cause of electrical fires in Norway. If the 2 structures were or are interconnected through a common ground/bond wire a circuit breaker would trip eliminating the difference in potential between the 2 structures.

This ironic example is actually on real reason why we bond even at premises level.


Post #214445 brings up TT briefly when soil is dry :laughing:

Anyway, about Norway:

http://www.electrical-contractor.net/forums/ubbthreads.php/topics/209335/1.html
 
#13 ·
Welcome :) I find it fascinating as well, it gets me to think about grounding, earthing and bonding in new ways. A lot of these systems evolved over decades of use; trial/error from my understanding. But a lot of it has been refined over the years through statistics and theoretical analysis, much like our codes and standards.




TT is new to any American EE or sparky. However, the only difference between TT and TN/our systems is a main bonding jumper. A ground bar still exists in the panel, all circuits have an EGC and water, UFER, plumbing, gas pipes ect are all bonded to the panel ground bar as well. The difference begins in that earth rods or plates are added which ensure conductivity to the soil.
So it begs the Q, are they monitoring / metering the ground impedance?

I'm aware some states require it here, but in said installs it becomes something of a more serious concern....



They may be 50ma, 100ma, 500ma , even 1000ma (depending on a variety of factors like disconnect times, earth loop impedance, ect, ect). 3 schemes exist. One is having non RCD mains and having all circuits on individual 30ma RCD breakers. The 2nd option is standard branch circuit breakers but with an RCD main, 100 or 300 ma being typical for residential. The 3rd way is highly recommended by IEC sparkies: All branch circuit breakers are 30ma RCDs with a 100ma delayed trip RCD main. Should either RCD fail, one will back up the other. A delayed trip main is used to allow for selective coordination so a fault on a branch circuit with a functioning RCD will not pop the main.
As i understand this (shaky at best) the 3rd IEC recommended version may be a 3 ph panel , with a 3 ph RCD , down to a number of din rail mountable 'sub panels' (in our terms) consisting of other lesser 'magnitude' (their term) RCD's

I've often thought this a premier install , something of a debatable point for the 'main afci' camp here to consider...




And EGC is still needed from exposed metal parts to the panel ground bar. Equal potential bonding is still done within the structure and considered essential to safety. GECs serve a 2nd purpose beside lightning protection: they act as EGCs. The soil is used as conductor to clear a fault. Because the resistance is over 10 ohm 99% of the time, an RCD is used to clear the fault.

Well they're dealing with milliamps, even if it is 1000 Meadow. Of course we know it take 4-6 ma to assume Vfib , but one really needs to make a good hand/hand or hand/foot thru heart connection for that.

Makes me wonder if Herbert Ufer delved into soil conditions, ph, moisture , bedrock, etc

These systems are very common in some countries, for example rural UK. TT has an advantage that when no other metallic paths exist between the customer and other customers/transformer fault currents are extremely limited, behaving almost like a high resistance grounded system. No stray voltage or ground currents exist which can be a big help with sensitive electronics.
Interesting tradeoffs . While it's obvious the TT wouldn't be good on our mutual municipal h20 systems , the isolation would be purer. Do we make our own high R monitored deltas to a water system?



In some cases TT earthing is mandatory over TN-C. :eek: One such example are caravans (mobile homes/trailers to us). Here only TN-S or TT can be
used. If TN-C was used and the PEN broke, the metal frame would energize up to 230 volts between it and earth. This is actually why mobile homes in the US always required a 4 wire feed and required 4 wire stoves and dryers well before the 1996 NEC.
all i can think of is 'bird on a wire'....


BTW, Im sure Mr. Chicken Steve is wondering: How was TT earthing accomplished before RCDs? Well, I have an answer for the.: VOELCBs

Starting at post #24, going into detail at 29:


http://forums.mikeholt.com/showthread.php?t=165856

Voltage Operated Earth Leakage Circuit Breaker.
as stated, not exactly fail safe, i bet there's a bit of squabbling over the pond on that one.....:whistling2:







PEN is the cheapest, and simplest, but has 2 down falls: one being open neutral the other constant current through the earth/metal parts. Personally IMO the best system is TN-S, but out of all the systems on earth it is the least used for services. The reason being cost. POCOs don't want to run an extra conductor, and would much rather let the customer pick an RCD or main bonding jumper.

Yet the poco's , as Mac alludes, utilize all our GE's as a neutral to clear high power faults, like an SWER.....

Norway used IT for decades, but got it very wrong when they did not (failed to) interconnect all grounding systems between homes through a ground wire on the pole. When an IT system faults it turns into a TN, or in this case a TT system. When a ground fault occurs in another structure on an opposite phase 230 volts potential now exists between 2 structures. Both a serious fire and electrocution hazard. It is rumored to be a major cause of electrical fires in Norway. If the 2 structures were or are interconnected through a common ground/bond wire a circuit breaker would trip eliminating the difference in potential between the 2 structures.

This ironic example is actually on real reason why we bond even at premises level.


Post #214445 brings up TT briefly when soil is dry :laughing:

Anyway, about Norway:

http://www.electrical-contractor.net/forums/ubbthreads.php/topics/209335/1.html
[/QUOTE]

I can only imagine those Nordic sparkys have their own diagnostic methods Meadow....

~CS~
 
#14 ·
So it begs the Q, are they monitoring / metering the ground impedance?

I'm aware some states require it here, but in said installs it becomes something of a more serious concern....
Yes, when the system is new. They do earth fault loop impedance testing and make sure the current passing through is over the RCD threshhold. However, as time goes on, I can only guess if the value is guareanteed (its not in reality) to stay the same.


As i understand this (shaky at best) the 3rd IEC recommended version may be a 3 ph panel , with a 3 ph RCD , down to a number of din rail mountable 'sub panels' (in our terms) consisting of other lesser 'magnitude' (their term) RCD's
Yup, RCD main and RCBO (combination breaker and GFCI) branch breakers often in the same enclosure. Best way to do a TT if you had to, though not required in most cases.




I've often thought this a premier install , something of a debatable point for the 'main afci' camp here to consider...
Having an main RCD, or at least 2 is very cheap compared to all RCD branch breakers. In the UK most new installs have 2 RCD mains. Basically one RCD controls half the home circuits with down stairs sockets and upstairs lighting While the other RCD controls the other half with upstairs sockets and down stairs lighting. Thus, if any one RCD trips, at least the other circuit may provide light from table lamps or vis versa cieling lights.





Well they're dealing with milliamps, even if it is 1000 Meadow. Of course we know it take 4-6 ma to assume Vfib , but one really needs to make a good hand/hand or hand/foot thru heart connection for that.

Makes me wonder if Herbert Ufer delved into soil conditions, ph, moisture , bedrock, etc
Me to. But a UFER is 2 birds in one stone. UFER not only goves true equal potential but its the best ground rod on earth, perhaps better than a well casing.

Europe RCDs are more than ours, but I think thats because of circuit size and nusiance tripping. Keep in mind our AFCIs are 30ma when detecting ground faults.





Interesting tradeoffs . While it's obvious the TT wouldn't be good on our mutual municipal h20 systems , the isolation would be purer. Do we make our own high R monitored deltas to a water system?
Indeed, a big trade off. Basically TT is a trade off of TN-C and TN-C is a trade off of TT when compared to TN-S which is the most advanced version of both.

TT only wins over TN-S and TN-C where arc flash may be a concern. But,
that assumes no other paths back to the source. IT gurantess low fault current in all cases because the neutral is either floating or resistored.





all i can think of is 'bird on a wire'....
Yup. Of note mentioned on ECN is that if the room had no conductive metal objects near by 2 wire non groudning sockets where allowed.






as stated, not exactly fail safe, i bet there's a bit of squabbling over the pond on that one.....:whistling2:
:laughing::eek: You can say that again. Ironically the voltage breaker requires isolating the water line whuich might help in urban areas should the
VOELB fail.









Yet the poco's , as Mac alludes, utilize all our GE's as a neutral to clear high power faults, like an SWER.....
POCO utilize them for 2 major reasons: free lightning protection, and a free second neutral conductor. Having a second neutral reduces the issues that are cropping with MGNs: ones that are getting undersized causing stray voltage; MGN breaks are masked by city water lines; and fault current is increased back to the substation which can be of benefit (for example a hard neutral to ground fault down the lines cuases voltage to rise on L-N transfomers connected to the other 2 phases from neutral shift {picture resitor in the neutral}. Issue can also occur from voltage regulators creating a neutral shift. [I will see if I can find the link] By having another noodle that voltage rise is reduced maening less reports of damaged equipment)


[/quote]

I can only imagine those Nordic sparkys have their own diagnostic methods Meadow....

~CS~[/quote]

:laughing: You should see the DIY forums (although technically agiast the law to be fixing your own electric in Norway) Same goes for the Sparkies. It gets confsing when neighbors are fighting of intermitment blown fuses.
 
#15 ·
Here is the link to neutral shifts. Basically the more neutral paths the better:


http://www.eng-tips.com/viewthread.cfm?qid=370493

"It is unusual in a distribution feeder to have a relatively high impedance neutral like alumoweld. I would expect maybe #4/0 AAC to be used with 336.4 kcmil AAC phase conductors. If you have much unbalanced load, then you will get excessive voltage drop (and losses) in the neutral. Neutral voltage drop will cause phase angle shifts. Voltage regulators will make the Ø-n voltage magnitudes equal, but will not change the voltage angles and you will have unbalanced Ø-Ø voltages."
 
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