Bonding TT and TN-S

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Sahib said:
The reason is a s?ngle phase system from high resistance grounded three phase system should not be used. I think there is a code prohibition for that.
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You can tape single phase loads from a 3 phase high resistance, however when it comes to medical application isolated power the NEC states it must be totally ungrounded except for the line isolation monitor.








Tony S said:
There are many forms of IT.

As has been mentioned medical applications (BS7671 has a whole new section to cover that).
In my case I?ve been involved with them on 11 and 3.3KV distribution where NER?s are used to limit earth fault currents.
Induction furnaces, the conductors and coils are water cooled so to provide a solid earth would be disastrous.
Mines and quarries, again to limit earth fault currents. (BS7671 doesn?t have a section for that).

But they aren?t used for general distribution. Which I believe is what the OP was asking about.
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Not all inclusively. Most of the world doesn't use IT, but there are a few areas of the world where it is/was exclusive for domestic supplies.
 
Tony S said:
Outer Mongolia aside, pray tell where these unearthed supplies are used for domestic supplies?
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Norway and very, very extensively (something like 95% of all domestic supplies). Norway is considered to have some of the best electrical safety standards in the world.
 
GoldDigger said:
Does Norway mandate ground detection for residences?

Tapatalk!
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In older installs no (everything was just "diazed" fuses):happysad:. Because of this its not uncommon for older systems with fuses to have standing ground faults, which defeats the purpose. However, newer installations or panel upgrades require directional RCD (GFCI) breakers on all circuits to clear any ground faults. All breakers are double pole common trip with GFI logic built in. Typical service is 3 phase with a 63 amp main.



Here a few examples of typical Norwegian load centers. First 4 have modern RCD breakers, the second one also shows the incoming power cable to the left (3 phases plus a ground no neutral), last one is an older diaze fuse version. Meters are in the panel to boot:
 
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Sahib said:
No, unless you use an isolation transformer.
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There would be no reason to use an isolation transformer. The isolation transformer would still be a single phase load connected to the system, adding a 3 phase transformer would just give you what you already have. Both would be unnecessary. Whether the load only takes 2 phases or 3 is irrelevant to it or the systems ability to function. Now if you wanted a TN system (grounded phase or leg) you would then have to use an isolation transformer, but not for a single phase load.
 
Suppose a person is changiing a bulb in a single phase system derived from a high resistance grounded three phase after switching off the single pole switch to the bulb The exposed neutral to the bulb can become live if there is a ground fault in the system.
 
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Which is one reason that the NEC in most cases prohibits having an ungrounded system when there is a neutral and requires that neutral to be the grounded conductor.

Tapatalk!
 
Sahib said:
Suppose a person is changiing a bulb in a single phase system derived from a high resistance grounded three phase after switching off the single pole switch to the bulb The exposed neutral to the bulb can become live if there is a ground fault in the system.
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In that case correct. With a standard Edison screw base socket or table lamp that is a legitimate problem. However in most other applications its usually not considered an issue. High bay, wall pack, shoe box HID fixtures in stores, warehouses, factories are frequently fed via 2 hots even on 480Y277 volt systems. In Canada it can be done as long as the ballast is a CWI or 3 coil regulator type to provide isolation. But in any case its assumed qualified professionals will be servicing these fixtures.


Euro type sockets usually aren't a problem since they are designed to shield the energized threads. The case for that being 2 prong schuko plugs can be reversed simply by unplugging and rotating the plug. No polarization exits.








GoldDigger said:
Which is one reason that the NEC in most cases prohibits having an ungrounded system when there is a neutral and requires that neutral to be the grounded conductor.

Tapatalk!
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Correct. But nothing in the NEC prohibits luminaires to be fed by 2 hots.:p:)
 
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