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Exploring the Use of RCDs in TN-C Installations: Old Buildings, Two-Wire Power Supply, and More

kryst16 29553 47
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Can an RCD be used safely and effectively in an old TN-C installation with a PEN conductor, or is it prohibited because it may be dangerous?

An RCD is not considered suitable for protection in a TN-C installation, because the PEN conductor acts as the protective conductor and must remain continuous; interrupting it through an RCD violates that rule [#16471746] Standard 4-41 is cited as categorically prohibiting RCD use in TN-C, and several replies state that in this system it does not provide electric-shock protection as intended [#16472334][#16471638] In practice it can trip on equipotential or leakage currents from appliance housings even when nothing dangerous is happening, so it may give a false sense of security [#16471904][#16471713] It can also fail to protect in some fault scenarios because the device only compares outgoing and returning current, not what happens beyond the socket [#16471904][#16471638] The thread’s consistent recommendation is to modernize the installation to TN-S and use the RCD there, rather than relying on it in TN-C [#16472539]
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  • #31 16472262
    Brivido
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    In this case, he should disconnect the switch, so I don't know what's going on. Well, unless it is about applying voltage from the outside, who knows, maybe the RCD will turn off before sitting on it :)
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  • #32 16472294
    aare
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    15kVmaciej wrote:
    Install the RCD in the TN-C installation with zeroing, apply voltage to the washing machine's casing and place the bare d% on it, taken straight from the shower.
    .... after all, it's only 30mA, it will definitely not feel anything .... :D


    Gentlemen! First of all, I suggest not to use ad personam arguments. Such "arguments" only testify to those who use them.

    Returning to the TNC and RCD installations, let me emphasize that:

    - the installations need to be converted into TNS, and all adding the RCD to the TNC is for the sole purpose temporary improved security

    - The RCD used in the TNC and installed in the fuse box will not work in certain situations (as I wrote about in the previous post). Hence my proposal to supplement the protection of sensitive devices (requiring a "pin") by the RCD in the socket

    - when installing the RCD, no one wrote about the abandonment of other means of protection. In other words no one wants to throw it away e.g. eSek etc.

    - we are aware of the possible tripping of RCDs in TNC installations due to leakage from the housing

    - The RCD will only work if the patient is electrocuted. It is supposed to work anyway, after all, it is an old installation that we want to slightly tune. The seat belts in the car also do not prevent an accident. The belts only mean that the brain, in most cases, is still inside the skull, not outside of it. Similarly, the RCD is supposed to increase the probability of the power being turned off while the paralyzed patient is still alive.

    - the best protection is common sense. However, there can always be a child sticking a wire into the socket, or some other deborer landing in the fog with a shout of Ku..a Mać. The RCD in the TNC is only intended to increase safety, but will not guarantee anything 100%


    In my life, I have already encountered a voltage on the refrigerator in the TNC installation. RCD in this situation would increase safety. I also met a child (hehe my own) who is just waiting for his parents to go out to do what he was forbidden to do. The use of security features that, although not perfect, but increase the level of security, makes sense in my opinion.
  • #33 16472308
    opornik7
    Electrician specialist  
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    aare wrote:
    First of all, I suggest not to use ad personam arguments.

    Dear colleague. There are no dilemmas for normal electricians. Further discussion does not make sense in my opinion.
    aare wrote:
    Coming back to the installation of TNC and RCD

    There is nothing to go back to or waste time writing it has already been written.
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  • #34 16472310
    3301
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    Brivido wrote:
    In this case, he should disconnect the switch, so I don't know what's going on. Well, unless it is about applying voltage from the outside, who knows, maybe the RCD will turn off before sitting on it :)

    It will not turn off, the washing machine stands on insulated adjustment screws (maybe not all of them) and if you try to sit down, you will get an electric shock and maybe turn off when the current is about 30mA
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  • #35 16472331
    Anonymous
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  • #36 16472334
    kkas12
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    Standard 4-41 categorically prohibits the use of RCDs in the TN-C system. This is the opening and closing record of the thread.

    You have the right to decide who can survive and who can not. Only your "law" is based on randomness, because none of you "rationalizers" knows the "predispositions" of a man who are hurt.
    Posts suggesting that the RCD limits the shock current are nonsense. It only aims to shorten the time it takes for it to pass through the body. And what does this current depend on, Maciek explained.
    But understand that these are too high thresholds for "rationalizers".

    Today, protection against electric shock is based on the fact that the first fault is to turn off the power.
    So when the voltage appears on the housing, the condition of the SWZ is to be met within a certain time. And it is definitely not 0.4 s in the case of sockets.
    ..
    Do you want to modernize old installations? I have nothing against it.
    But it cannot be based on the use of modern solutions in installations that do not exist today.
    They killed about ten people per million inhabitants of this country every year !!!
    Today, when PE and N were separated, this ratio has dropped to about 3.5 deaths per million inhabitants.

    So do not turn back with the Vistula stick, because for me your theories are the reincarnation of the late friend Krystyna.
    And I am not going to eat this frog again, even though I appreciated it very much.
    Even when we disagreed.
    He was classy and knowledgeable.
    Unfortunately, you neither have his class nor his knowledge.
    You only have a vision.
    You just think something.
  • #37 16472341
    zbich70
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    opornik7 wrote:
    There is nothing to go back to or waste time writing it has already been written.

    Hundreds of times on all electrical forums, just look at Matołek .
    The dilemmas and arguments of electric voodoo supporters have long been dispelled and refuted.
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  • #38 16472383
    tantalos1
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    And this is a textbook example: a woman in a bathtub dries her hair with a dryer and the dryer falls into the bathtub. The installation has only zeroing because ... it cannot have an RCD.
  • #39 16472403
    opornik7
    Electrician specialist  
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    tantalos1 wrote:
    a woman in the bathtub dries her hair with a dryer and the dryer falls into the bathtub

    And writing for writing begins.
  • #40 16472406
    Strumien swiadomosci swia
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    tantalos1 wrote:
    a woman in the bathtub dries her hair with a dryer and the dryer falls into the bathtub


    And where does 230V come from in the bathtub area? The dryer had a 3 meter cord? Which hair she was drying, why her hands were slick, a topic for S. Holmes.
  • #41 16472423
    tantalos1
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    Strumien swiadomosci swia wrote:
    tantalos1 wrote:
    a woman in the bathtub dries her hair with a dryer and the dryer falls into the bathtub


    And where does 230V come from in the bathtub area? The dryer had a 3 meter cord?

    In old apartments, there are various items close to the bathtub, such as a washing machine. We are not talking about new installations, but about old ones.
    Let me give you another real-life example: in the well there is a pump for watering the garden, the pump has a metal casing, but it is in protection class II and has a plug without a pin. After a short time, it started to knock out the RCD, after examining it, it turned out that it had damaged insulation of the power cable. Without the RCD, no one would have realized that something was wrong and that they could electrocute someone.
  • #42 16472424
    Anonymous
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  • #43 16472437
    kkas12
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    Standards were also applied in "old" flats. They were all the more used that they were obligatory.
    The pump was earthed through its foundation so it blew out. If it wasn't for the voltage on the housing, it would be waiting for you.
    Will you trust the camera?
    And it is one of the most unreliable cameras, as many factors affect this unreliability.
  • #44 16472471
    CYRUS2
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    15kVmaciej wrote:
    Install the RCD in the TN-C installation with zeroing, apply voltage to the washing machine's casing and place the bare d% on it, taken straight from the shower.
    .... after all, it's only 30mA, it will definitely not feel anything .... :D

    Does your friend have any concept of TN-C?
    Let a colleague give the phase for the washing machine casing in TN-C.
    A colleague will find out what the results will be.
  • #45 16472504
    Brivido
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    Why are they currently used in TN-S?
  • #46 16472519
    aare
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    15kVmaciej wrote:
    This woman is a blonde who has not been able to understand how to handle electrical devices - non-technical protective measures, you are starting to appreciate their importance ????!


    A colleague wrote about a woman with a hair dryer, someone wrote back: where did he get such a long cable, then speculation about hair colors ... everything to avoid a substantive answer.

    A real-life example. Hot zero, 220V (it was 220 then) on the refrigerator. About 30 cm from the central heating installation, about 50 cm from a metal stove connected to the gas installation. In this case, it is safer without an RCD ????????

    I will use an analogy to road traffic. When I went to Sweden for the first time several years ago, I saw some interesting things:
    - Swedes traveled like "haters" - a matter of upbringing and awareness of threats
    - many roundabouts, lanes separated by fences, narrowing of the road near stops - if someone suffers, it is not possible to commit stupidity
    - openwork lighting poles, energy-consuming barriers - if someone suffers someone and manages to do something stupid, we minimize the effects

    All this on roads comparable to our countries. They also don't have the money to build collision-free roads everywhere.

    Likewise, it is not always possible for us to immediately replace TNC with TNS. The TNC is used in millions of homes and it is unacceptable to accept these 10 victims per million inhabitants until the complete replacement with the TNC (which will probably come in decades) is unacceptable.
  • #47 16472522
    CYRUS2
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    Brivido wrote:
    Why are they currently used in TN-S?
    Less failure rate.
    PE conductor failure is not a threat.
    The failure of the N conductor is also not a threat.
    The failure of the N + PE conductor is also not a threat.
  • #48 16472539
    Akrzy74
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    kryst16 wrote:
    There are hundreds of thousands of houses in Poland where 20 nests work on one phase with the old "plug" and there are no problems with it,

    It depends on who and what considers the problem. For some, the problem is, for example, a situation when, during an overload (or short-circuit), the overload switch is activated and replaced with a larger one, disregarding the fulfillment of the trip condition (they have not even heard about it), for others, e.g. the situation of connecting a device with damaged insulation to the installation which uses an RCD and due to the principle that the device must work, because it worked in an installation without an RCD, it is therefore necessary to "bridge" or bypass the residual current device.
    Thousands of posts have been written on similar topics (not only on this forum), so I will not tolerate "forum fairy tales" by my friend as an electrician. Please rely on the statements of electricians and we will avoid misfortune. An RCD is not a panacea for everything. Everyone who knows the operation of this apparatus knows this and knows where to use it, where to avoid it and for what reasons. In this section, it should be enough information that the camera cannot be used in a given system (end!).

    The topic is closed. This is a forum for serious and sensible people.
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Topic summary

✨ The discussion revolves around the use of Residual Current Devices (RCDs) in TN-C electrical installations, particularly in old buildings with two-wire power supplies. Participants express concerns regarding the safety and effectiveness of RCDs in such systems, highlighting that while RCDs can provide some level of protection against electric shock, they may not function as intended due to the nature of TN-C systems. Key points include the risk of disconnecting the PEN conductor, the potential for false security, and the inadequacy of RCDs in preventing electric shocks when devices are improperly connected. Some argue for the necessity of RCDs as a temporary safety measure, while others emphasize adherence to standards that prohibit their use in TN-C installations. The conversation also touches on the importance of proper grounding and the limitations of RCDs in protecting against electrical faults.
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FAQ

TL;DR: Approx. 35 % of pre-1990 Polish dwellings still run on two-wire TN-C networks [GUS, 2020]. "RCD does not meet the requirements necessary to operate fully as intended" [Elektroda, Turysta_, post #16471713] Standards forbid using an RCD as primary shock protection in TN-C [IEC 60364-4-41].

Why it matters: Homeowners often add RCDs to legacy wiring, unaware of hidden failure modes.

Quick Facts

• IEC 60364-4-41 §411.4.4 bars RCDs as sole shock protection in TN-C [IEC 60364-4-41]. • A 30 mA RCD must disconnect within ≤200 ms at rated residual current [Elektroda, 15kVmaciej, post #16471840] • Typical Polish retail price for 2-pole 30 mA RCD: PLN 80–120 [Ceneo, 2023]. • B10 MCB needs loop impedance ≤11.5 Ω to trip in 0.4 s; C16 needs ≤2.9 Ω [IEC 60364-4-41, Annex A]. • Reported fatal electric-shock rate fell from 10 to 3.5 deaths / million after TN-S adoption [Elektroda, kkas12, post #16472334]

1. Why do experts discourage RCDs in TN-C wiring?

The PEN conductor in TN-C serves as both neutral and protective earth. An RCD opens this conductor together with phase, breaking the continuity that standards demand for a protective path [Elektroda, Voldi123, post #16471746] If the device fails while PEN is open, its metal case can remain live until the user completes the fault path, defeating “first-fault” protection [IEC 60364-4-41].

2. Is installing an RCD in TN-C illegal?

Polish and IEC rules do not explicitly forbid adding an RCD, but IEC 60364-4-41 §411.4.4 states it cannot count as basic or additional shock protection in TN-C. Authorities may therefore reject designs that rely on it alone [IEC 60364-4-41].

3. What happens if the PEN conductor breaks upstream of an RCD?

A broken PEN shifts the device housings to full phase voltage; the RCD sees balanced current and stays closed [Elektroda, Voldi123, post #16471897] Users may touch 230 V without disconnection—an edge-case many homeowners overlook.

4. Does an RCD limit shock current?

No. Shock current depends on body resistance and voltage. The RCD only shortens exposure time; a 30 mA unit may still allow >100 mA for up to 200 ms before tripping [Elektroda, 15kVmaciej, post #16471840]

5. Can plug-in socket RCD adapters improve safety in TN-C?

They help only when the fault current returns through the adapter’s N conductor. If the fault uses pipes or floors as return, the adapter sees imbalance and trips, giving partial protection [Elektroda, aare, post #16471748] They do not protect against PEN breakage.

6. Why do RCDs nuisance-trip in mixed two-wire systems?

Appliance cases may sit at a different potential than heating or water pipes. Touching both creates a leakage path that the RCD detects as residual current, even though no insulation fault exists [Elektroda, Strumien…, post #16471904]

7. What is the recommended upgrade path for old flats?

  1. Split PEN into separate PE and N at the service head (TN-C-S).
  2. Re-wire final circuits with three-core cable, bonding all exposed metal.
  3. Add 30 mA RCDs or RCBOs per circuit [IEC 60364-4-41]. This sequence restores uninterrupted PE while enabling modern residual protection.

8. How does over-current protection compare with RCDs for shock safety?

A B10 MCB trips only when fault current exceeds about 50 A within 0.4 s [IEC 60364-4-41, Annex A]. Such high current rarely flows through the human body, so MCBs protect mainly against fires, not direct contact. RCDs act at milliamps, targeting shock exposure.

9. How can I test an RCD in a two-wire circuit safely?

  1. Plug an RCD tester that diverts 30 mA from phase to a dedicated earth rod.
  2. Press the test button; confirm trip within 200 ms.
  3. Repeat monthly. Note: this test bypasses the PEN, so it does not reveal upstream break risks [Manufacturer manual].

10. Will an RCD save a child inserting a nail into a two-wire socket?

Not reliably. Contact between phase and neutral inside the socket causes a short that trips the MCB, but the child still experiences initial shock. If the nail touches only phase, no residual current flows; the RCD stays closed [Elektroda, masonry, post #16471638]

11. Are two-pole RCDs available that keep neutral closed?

Manufacturers avoid such designs because leaving neutral connected while opening phase complicates EMC filters and risks half-powered equipment fires. Standard RCDs therefore open both poles [Elektroda, Brivido, post #16471566]

12. Can heating or water pipes serve as protective earth?

No. Corrosion, plastic sections, or isolation valves break continuity. Using them as PE violates IEC 60364 and can create lethal touch voltages if PEN fails [IEC 60364-5-54]. “Ground through radiator” scenarios in the thread show why RCDs trip unpredictably [Elektroda, Voldi123, post #16471897]

13. What statistic links TN-C to fatalities?

Before PE/N separation, Poland recorded about 10 electrocutions per million residents annually; the figure fell to 3.5 after TN-S upgrades [Elektroda, kkas12, post #16472334] This change underlines the benefit of modernising rather than patching.

14. Why do standards still mandate RCDs in TN-S but not TN-C?

In TN-S, the protective conductor remains continuous when the RCD opens neutral, so exposed parts cannot rise to full phase voltage. TN-C lacks that separation, so the same action can leave metalwork live, breaching the “automatic disconnection of supply” principle [IEC 60364-4-41].

15. What are common failure modes of an RCD itself?

Contacts may weld, mechanics can stick, and sensitivity can drift. Routine tests show up to 6 % failure after five years in harsh environments [ZVEI, 2018]. "Nothing is 100 % guaranteed" [Elektroda, opornik7, post #16471542]

16. Does adding an RCD ever increase risk?

Yes. Users may assume full protection and ignore dangerous two-wire sockets, continuing to plug Class I devices without earth. This false sense of safety can end fatally if PEN breaks and no upstream bonding exists [Elektroda, Turysta_, post #16471713]
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