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gokmen
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A Few Questions About RCDs

06/26/2014 5:47 PM

1) I am aware that IEC has specified an upper limit of 300mA for RCDs intended for fire protection, and some tests have shown that a 300 mA fault current can cause a real risk of fire. However, there are also residual current protective devices with tripping current greater than 300mA; such as 500mA, 1A and even 3A. I have read many standards, but could not find where or when an RCD with a residual current sensitivity rating of more than 300mA is allowable. And also, let's say, a socket-outlet is protected with an RCD with a trip range of 30mA, the sub-panel feeding the socket-outlet is protected with an RCD with a trip range of 300mA, so what can I use to protect the main panel, when I can only use an RCD with a residual current sensitivity rating of not more than 300mA to provide protection against fire risk?

2) I have seen many single line diagrams, sometimes where no RCD is shown on single line diagram, while other times each circuit is protected with an RCD, and there are also projects where only socket-outlets are protected with an RCD, while lighting circuits are not. Some people told me that some circuits are regarded as "critical" such as lighting and unlikely to be touched by people, thus they are not protected with an RCD, or an RCD with a residual current sensitivity rating of 300mA to provide protection against fire risk, and which could also protect human life on some occasions; however, I could not find anything related to that in any standard either.
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SolarEagle
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#1

Re: A Few Questions About RCDs

06/26/2014 11:59 PM

"

Sensitivity[edit]

RCD sensitivity is expressed as the rated residual operating current, noted IΔn. Preferred values have been defined by the IEC, thus making it possible to divide RCDs into three groups according to their IΔn value.

  • High sensitivity (HS): 6 - 10 - 30 mA (for direct-contact / life injury protection)
  • Medium sensitivity (MS): 100 - 300 - 500 - 1,000 mA (for fire protection)
  • Low sensitivity (LS): 3 - 10 - 30 A (typically for protection of machine)

Note that the nominal value of residual current indicated above is not an absolute value. More information is presented below.

Type[edit]

Standard IEC 60755 (General requirements for residual current operated protective devices) defines three types of RCD depending on the characteristics of the fault current.

  • Type AC: RCD for which tripping is ensured
    • for residual sinusoidal alternating currents
  • Type A: RCD for which tripping is ensured
    • as for type AC
    • for residual pulsating direct currents
    • for residual pulsating direct currents superimposed by a smooth direct current of 6 mA, with or without phase-angle control, independent of the polarity
  • Type B: RCD for which tripping is ensured
    • as for type A
    • for residual sinusoidal currents up to 1 kHz
    • for residual sinusoidal currents superposed by a pure direct current
    • for pulsating direct currents superposed by a pure direct current
    • for residual currents which may result from rectifying circuits
      • three pulse star connection or six pulse bridge connection
      • two pulse bridge connection line-to-line with or without phase-angle monitoring, independently of the polarity

Break time[edit]

There are two groups of devices:

  • G (general use) for instantaneous RCDs (i.e., without a time delay).
    • Minimum break time: immediate
    • Maximum break time: 200 ms for 1× IΔn, 150 ms for 2× IΔn, and 40 ms for 5× IΔn
  • S (selective) or T (time delayed) for RCDs with a short time delay (typically used in circuits containing surge suppressors)
    • Minimum break time: 130 ms for 1× IΔn, 60 ms for 2× IΔn, and 50 ms for 5× IΔn
    • Maximum break time: 500 ms for 1× IΔn, 200 ms for 2× IΔn, and 150 ms for 5× IΔn

Surge current resistance[edit]

The surge current refers to the peak current an RCD is designed to withstand using a test impulse of specified characteristics (an 8/20 µs impulse, named after the time constants of the rise and fall of current).

The IEC 61008 and IEC 61009 standards impose the use of a 0.5 µs/ 100 kHz damped oscillator wave (ring wave) to test the ability of residual-current protection devices to withstand operational discharges with a peak current equal to 200 A. With regard to atmospheric discharges, IEC 61008 and 61009 standards establish the 8/20 µs surge current test with 3 kA peak current but limit the requirement to RCDs classified as Selective."

http://en.wikipedia.org/wiki/Residual-current_device

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Blackpaper
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#2

Re: A Few Questions About RCDs

06/27/2014 2:57 AM

The panels don't need RCD protection everywhere.
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Crabtree
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#3

Re: A Few Questions About RCDs

06/27/2014 4:32 AM

Which country are you in, Bwana? Have you read the local electrical standards there, by any chance?

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Blackpaper
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#5
In reply to #3

Re: A Few Questions About RCDs

06/27/2014 6:38 AM

I work in this field. The panel itself do not need RCD protection
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gokmen
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#8
In reply to #5

Re: A Few Questions About RCDs

06/27/2014 9:12 AM

The panel should also be protected by an RCD. If there is a leakage current from the panel or the cable feeding the panel to the surrouding area, then it can pose a fire or electrical shock risk too.
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gokmen
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#7
In reply to #3

Re: A Few Questions About RCDs

06/27/2014 9:07 AM

I most certainly do not live in France which has its own unique standars like NFC 17 102 recognized by no other country. Most of the countries in the world recognize either IEC standars or American standards, it is the same in Turkey where I live as well.
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TonyS
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#9
In reply to #7

Re: A Few Questions About RCDs

06/27/2014 8:30 PM

I think the humour in Crabtree's nom de plume and avatar is lost on non UK residents.

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dkwarner
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#10
In reply to #9

Re: A Few Questions About RCDs

06/27/2014 11:42 PM

Correct!

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WJMFIRE
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#12
In reply to #9

Re: A Few Questions About RCDs

09/09/2014 2:46 PM

Not all non-UK residents.

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navdeep_goyal
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#4

Re: A Few Questions About RCDs

06/27/2014 6:13 AM

For knowing about RCD'S visit link below:-

http://electrialstandards.blogspot.in/2014/05/residual-current-detector-principle-of.html
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Blackpaper
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#6
In reply to #4

Re: A Few Questions About RCDs

06/27/2014 6:42 AM

Who is talking about the working principle here?
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ade_jkrs
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#11

Re: A Few Questions About RCDs

07/04/2014 12:30 AM

Hi,

If you're connected to TT system RCD protection is paramount for shock protection (save for medical equipment where protection schemes are specific) as earth fault loop impedance is high. RCDs for equipment subject to frequent personal contact eg those connected to switch sockets - 30mA RCD; lighting, air-conds (not frequently touched except by electricians during repair) - 100mA.

Where RCDs are "cascaded" (more than 1 in a circuit), discriminate by time delay and sensitivity eg. 30mA Type G instantaneous downstream, 300mA type S with 50mS delay mid-stream, 1A type S with 250mS delay upstream. Discrimination by sensitivity alone is often guranteed to fail. Select type AC for largely linear loads, Type A for non-linear loads - VSDs, electronic ballasts, SMPS.

Do take note that where 3-phase RCD are used, RCD sensitivity for protection of life is still 30mA, not 100mA.

Other good installation practice - TT system: (i) use 2 or 4 pole control for compound lights - enables isolation of the circuit in the case of N-E faults (quite common for external circuits) so that your RCD can be turned back on; (ii) supply single phase loads from single phase RCD - the issue of desensitization of 3-phase RCDs due to unbalanced residual/standing leakage current on the 3 phases is a very real issue but never emphasized in technical literature.

Best regards.
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PWSlack
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#13

Re: A Few Questions About RCDs

10/17/2024 7:37 AM

One way of doing correct domestic <...RCD...> installation on TT in the UK is to have a 100mA device servicing the whole house, which is low enough to detect a circuit fault to the earth electrode, and then 30mA devices on downstream circuits to provide discrimination at the distribution board.

There are a number of ways of doing the latter under BS7671, one of which is to put one 30mA RCD upstream of everything except the circuit serving the domestic freezer, the lighting and the smoke alarm(s), which don't warrant it, and the other is to install 30mA RCBOs on the appropriate outgoing circuits; it tends to get a bit busy in the distribution board if doing the latter as they occupy more space than MCBs.

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Users who posted comments:

ade_jkrs (1); Blackpaper (3); Crabtree (1); dkwarner (1); gokmen (2); navdeep_goyal (1); PWSlack (1); SolarEagle (1); TonyS (1); WJMFIRE (1)

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