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.
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