I wonder why American buildings are mostly grounded TNCS and not just TNC.
They have virtually no RCD protection, and also an RCD could work fin on TNC system as long as PE on outlet is connected on PEN before the RCD (or just left not connected). Aren't they just wasting copper?

  • 6
    Seeing as this is a DIY site, frequented primarily by laypersons, it might be appropriate to expand your initialisms for us. I can't decipher any of them and I've been in construction for a very long time. :)
    – isherwood
    Feb 15, 2019 at 16:23
  • 1
    Yeah, I know a lot about grounding, know why streetcars and diesel locos have control systems grounded in such different ways, and maintain isolated-ground systems. And I have no idea what this is saying. So those terms are not commonplace, and links to their definitions would be vital. Feb 15, 2019 at 17:26

2 Answers 2


We do use RCD (Residual Current Device) protection, and rather extensively these days even, it's just hiding in disguise

North American electrical wiring indeed started off as a TN-C (all protective earthing is accomplished by referencing things to the neutral) system, back in the bad old days before we had three-prong outlets and GFCIs (Ground Fault Circuit Interrupters), and evidence of this can be seen to this day (such as the abhorrently common NEMA 10). However, two things happened:

  1. Unlike Europe, who was coordinating a brand-new standard for wiring installs "all at once" if you will through the IEC process, protective grounding (TN-C-S) and GFCI (RCD) protection were added to the US NEC (the US wiring regulations, more or less) separately; furthermore, the GFCI protection rules in the NEC were phased in, location by location.

  2. Our GFCI protection started off with wide adoption at the receptacle level due to the desire to retrofit shock protection into existing houses, especially in wet areas (bathrooms and kitchens). This is in contrast to the IEC style of RCD, which was widely adopted in breaker form due to the larger number of new installs in Europe.

The consequence of these two things is that the sensitive UL 943 Class A GFCI, while widely deployed in the US nowadays, winds up performing a very different function to IEC RCD protection -- the former is designed to protect against electric shock, including "can't let go" hazards up to and including shock-drowning, while the latter protects primarily against fire, with protection against gross shocks as a side-effect.

Fast-forward a while, though, and the US, while having largely tamed its electrocution problem, still has a serious fire problem, partially electrical in nature. This electrical fire issue is blamed on a variety of abuses and defects that cause what are described as "arcs", but are more precisely a surface-tracking type of fault where excessive current intermittently leaks through damaged insulation, slowly carbonizing/pyrolyzing it to the point where it smolders into an electrical fire.

As a result, electrical protection manufacturers in the US come up with what's called the Arc Fault Circuit Interrupter, or AFCI, that is designed to listen for the conducted-RF signature of these arc faults and break the circuit when one is detected. Due to a combination of design limitations and accidents of history, though, the early AFCIs were built off of what basically is a GFCI platform, and as thus had a 30mA equipment protection ground fault trip function in them that is used to catch arcs-to-ground, despite the functionality not making it into the UL standards.

As a result of the rapid push to adopt AFCI breakers, and the retention of this GFPE trip functionality by 3 of the 4 North American breaker manufacturers (GE is the only one to have dropped it), in many cases, a modern house in the US will have RCD protection at the 30mA level on most branch circuits, with select circuits protected at 6mA. However, this protection is provided at the level of individual branch circuits, not service-wide as is typically done in 5-continent power.

Why is this, you might ask? Well, service-wide ground-fault protection is applied to large, low-voltage three phase services in North America to protect against fires, and works well in those applications. However, the sensitivity of North American ground fault devices at residential-scale operating currents requires they use electronic ground fault sensing, as opposed to early IEC direct-acting RCDs that used the differential current in the measurement CTs to operate the trip coil without any sense electronics. This has the upside that North American GFCIs were able to achieve reliable performance at low leakage levels early on; however, it has the downside that wiring a GFCI up backwards (with power being fed in on the LOAD side) will break it in a non-obvious fashion.

As a consequence of this, you can't reasonably use a GFCI as a main breaker in North America unless you are willing to take the same approach as the large wye services do, otherwise you run into issues as soon as you start doing other sophisticated and desirable things, like solar power. Furthermore, the larger size of US services means that a central RCD would need to be excessively insensitive due to the simple fact the differential leakage currents are higher.


First of all. TN-C means neutral is used as a protective conductor. So when there's a fault to the metallic parts of a device, a short circuit occurs and a circuit breaker trips.

TN-C-S means that before the entry point (supply side) it's a TN-C system, relying on a short circuit (a fuse that's not very sensitive) to protect people; and after the entry point (consumer side) it's a separate PE conductor (third prong).

Neither TN-C nor TN-C-S specify where the protective devices (RCD) are located. They're only concerned about the fault path. In TN-C-S the protective conductor is located closer to the supply side. If there's a ground rod at the premises connected to PE it would be a TT system.

TN-C-S are much safer than TN-C because at the premises the PE conductor doesn't hold the load current. So if there's an open neutral you'll be still protected.

Now, Americans usually have RCD devices at the outlet. That's because they have 3 phase system and powerful devices require voltage between two phases. So it's much harder to install RCD at the breaker because RCD relies on the difference of current flowing to hot and neutral. There's always a difference of current flowing between two hot wires of different phases, so RCD won't work at the breaker. It would only work at the outlet.

  • Why wouldn't a RCD in the panel work? Three phase systems are common in Europe as well, and you get both 3P+N and 1P+N RCD breakers.
    – vidarlo
    Apr 14 at 19:35
  • US GFCIs do exist in breaker form and work just fine when used that way. Apr 14 at 19:38

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