TT and TN-system and RCD
TT and TN-system and RCD
(OP)
Hi,
We know that in TT System the neutral is earthed at the utility ground point and the consumer earth is seperate. In TN System the exposed-conducive parts are all solidly connected to the utility neutral-earth point.
Can we safely say that RCD is not mandatory for the protection of people for a TN system ?
Bob
We know that in TT System the neutral is earthed at the utility ground point and the consumer earth is seperate. In TN System the exposed-conducive parts are all solidly connected to the utility neutral-earth point.
Can we safely say that RCD is not mandatory for the protection of people for a TN system ?
Bob





RE: TT and TN-system and RCD
In fact, there are EU rules saying that public areas shall have RCDs.
30 mA RCDs are used to protect live beings from changing status into dead beings.
Gunnar Englund
www.gke.org
RE: TT and TN-system and RCD
Anyone shed any light?
Apologies for being barely "on-topic"
A.
RE: TT and TN-system and RCD
zeusfaber: According to the IEC rules, RCDs are not to be used on TN-C systems. If used in a TN-C-S (i.e. a combination of TN-C and TN-S), RCDs should ONLY be used in the TN-S part of the system, meaning after splitting the PEN-condutctor into one PE- and one N-conductor. As a summary, RCDs should not be used in systems with PE- and N-conductors combined (as for the TN-C).
skogsgurra/maypot: What EU-rules are saying that public areas shall have RCDs? Just curious, because I cannot find such a requirement in the IEC60364?
From my point of view, the RCD can be omitted in TN-systems as long as you can prove that the MCB will disconnect within a certain amount of time (depending on the MCB and the expected, minimum fault-current which in fact is set to 30mA with the RCD).
In TN-systems you want the fault current to be sufficiently large enough to trip the MCB (speaking several tens of amps). In IT-systems you try to minimize the fault-current in order to making it safe to work even with a fault, i.e. it depends on how good your earthing-resistance is (speaking of miliamsp). The TT-systems is somewhere in between, making it very difficult to avoid RCDs.
However, in practice, I agree that it is a good idea to use RCD in most public applications.
Have a nice day.
RE: TT and TN-system and RCD
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One day my ship will come in.
But with my luck, I'll be at the airport!
RE: TT and TN-system and RCD
We have the same thing in Sweden. I think that it is EU-wide now.
Gunnar Englund
www.gke.org
RE: TT and TN-system and RCD
All I can find in the IEC60364 is that you classify the room "due to external influence" (not sure of the correct, english term) as BA2 (Children). Thereby you require a specific IP-degree for electrical equipment, inaccessibility of electrical equipment and certain requirements of surface temperatures.
I know that only special socket outlets are allowed where children can access the outlets. This is to fulfil the IP-requirement, making sure you cannot put your fingers into the holes. Basically, in Norway, these outlets are used everywhere expect offices et.c nowadays.
Also, there is a clause claiming that (freely translated to english) "an installation with increased risc of electrical shock because of external influence, the maximum contact voltage is not to exceed 25V"...... Maybe this can be a subject to discussiong wheter or not kindergartens should have 25V (which is quite hard to achieve without RCBs)....?
RE: TT and TN-system and RCD
Your take on TNC-S is the same as mine. The rumour (mistaken, I think) was that you couldn't use RCDs in the TNS section either.
Cheers.
A.
RE: TT and TN-system and RCD
Basically only RCD can fulfill these requirements.
Bob
RE: TT and TN-system and RCD
413.1.3.3: It is said that Zs*Ia <= U0, i.e. system-impedancem multiplied by the minimum current that assures that the breaker is disconnected within a given time should be less than the nominal phase-voltage.
The time is given as a table, and for U0=230V (i.e. TN Ull=400V) the maximum time is 0,4 seconds.
So, if the minimum faultcurrent that occurs is quite large, and the system-impedance is kept quite low, you should be able to fulfill this requirement with a MCB, depending on it's caracteristics (B,C,D) and the calculations you do. The whole clue is to assure that the fault is limited and disconnected after a short time. You must assure that your MCB will disconnect in the electromagnetically (within the time) and not thermal area of its caracteristic curve. If the faultcurrent is too small to trip the breaker, then you've done something wring.
However; 413.1.3.4 says that the time requirement mainly goes for portable-,handheld equipment or equipment connected with a socket outlet........ As opposed to
413.1.3.5: For stationary equipment, i.e. mounted to the wall, and not supplied from outlets et.c., you can allow a time up to 5 seconds. Then the requirement is that the impedance in should not exceed 50V*Zs/U0, and there is to be connected an equipotential bonding concductor.
THEN 413.1.3.6: If the requirements from 413.1.3.3, 4 or 5 above cannot be fulfilled by using regular overcurrent protection (typically MCBs), one is supposed to use equipotential bonding concductor and/or RCD.
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So the real requirement is made to the system impedance, and to get rid of the fault within a specified time. Sometimes, you need a RCD to obtain this in a TN-system.