TT and TN-system and RCD

[maypot]

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

 

[Skogsgurra]

No. On the contrary. An RCD will sense fault currents caused by "someone or something" touching a live wire and being connected to ground at the same time.

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

http://www.gke.org

 

[zeusfaber]

There used to be a myth doing the rounds (may still be today) that the wiring regs prohibited use of RCDs on TNC-S (PME to the rest of us) systems. Such a ban never made a lot of sense to me, and I could never find anything in the regs to support the idea.

Anyone shed any light?

Apologies for being barely "on-topic"

A.

 

[torslum]

Not sure if we disagree, but just to clearify:

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.

 

[ScottyUK]

Good advice from torlsum.



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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[Skogsgurra]

I am not so sure about the "public areas" - I was thinking of kindergartens and such places. I haven't found the EU text, only the German one: "Für Kindergärten und Kindertagesheime sind Fehlerstrom-Schutzschalter (Nennfehlerstom 30 mA, maximale Berührungsspannung 25 Volt) vorgeschrieben". There is a reference to the German "Vorschrift" Unfallverhütungsvorschrift GUV 2.10

We have the same thing in Sweden. I think that it is EU-wide now.

Gunnar Englund

http://www.gke.org

 

[torslum]

Not to sure about that the kindergartens. In Norway, we use the Cenelec/EN and IEC-rules also, even if we are not a EU-member. But all countries can add their own, specific additions to such standards. Maybe this is the case, if there is a specific requirement of 30mA or 25V for a kindergarten in some countries?

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)....?

 

[zeusfaber]

Torslum,

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.

 

[maypot]

Torslum wrote 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 reality, the protection device in TN-system should be designed in case of fault to trip at rated fault current of 30 m A and in less than 40 mS as per IEC.
Basically only RCD can fulfill these requirements.

Bob

 

[torslum]

According to IEC60364, for TN-systems and protection against electrical shock:

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.