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enter image description here Hi,

I want to use the heating tape (constant wattage) and I was told that I should use RCD for the safety. I am new to RCD so I wanted to double-check if this is a correct way to wire the RCD with the power supply and the heating cable. Your help will be greatly appreciated. Thank you.

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  • Is there any other slot in the RCD for connecting the earth wire to it? – gusgus Sep 3 at 18:44
  • I think it doesn't have earth wire slot. this is the one i'm looking at ( uk.rs-online.com/web/p/rcds/8929485) – Tquestions Sep 3 at 22:47
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Note: You haven't mentioned your jurisdiction, but it is quite likely that you might need qualified/authorized electrician to perform electrical installation changes according to the Code.

That being said, yes, in your picture you connected RCD correctly.

RCD (also called GFCI or FID elsewhere in the world) is residual current device, which means it will measure the difference in current coming in from Live side and going out to Neutral side. If that current is bigger than its sensitivity (30mA in your case), it will break the connection. Single phase RCDs (as your in the picture) do not have ground connection (just 4 connections: Live in, Live out, Neutral in, Neutral out). See failure modes below on why.

Also note that "63A" marking on RCD is not any measure of its protection, it is simply maximum current rating that is capable of operating on. If you go over that, you will destroy your RCD (which you do not want).

Please note that you also need appropriately sized overcurrent circuit breaker (MCB) in addition to RCD (or get combined RCD/MCB in one package, sometimes called RCBO). For example, if your heating cable is 1000W, you would install 6A MCB (as that is first MCB size above 1000/240). If it is 2000W you would get 10A etc. In your picture, you would preferably connect it on L input before RCD.

You also need to monthly check your RCD by pressing "Test" button (yellow in your example) and verify that is tripping the RCD.

For those interested in details, let's see failure modes of your heating cable:

  • there could be very damaged isolation between L and N wires in it, causing very high short circuit currents, in which case almost any MCB will trip immediately due to its electomagnetic component. RCD would not help here, as all current coming in from L is going out thorough N.

  • there could be slightly damaged isolation between L and N wires in it, causing higher (but not extremely high) currents, in which case your MCB (if appropriately sized) will trip after some time (seconds to even minutes!) due to its thermal bimetal component. There could also be arcing involved is such case, where additional device called AFCI might help. RCD would not help here, as all current coming in from L is going out thorough N.

  • there could be damaged isolation between L and protective Ground mesh. In this case, current will go to protective ground cable. In this case, RCD will trip on very small current (0.03A leakage) as not all current that entered via L exited via N. If you did not have RCD, MCB would trip only if the isolation is totally gone (not on partial damage), and then only if grounding in your object was low-impedance (which may or may not be the case). Otherwise the damaged area would overheat, leading to fire hazard or melting isolation (and thus leading to other error modes).

  • there could be damaged isolation on L is such a way that is exposed but does not touch the protective Ground mesh (for example if mesh was stripped during bad installation). In that case, if you did not have RCD, and you touched the exposed L, you could die, as the current would flow from L, through your body, to ground on which you stand (especially if it was for example in bathroom, and you were wet), causing cardiac arrest. With RCD, the current also would start to flow through your body, but RCD would detect that not all current has returned via N and would trip in the matter of milliseconds, thus saving your life.

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No no no, you can't directly feed a branch circuit load off the pictured 63A breaker!

A breaker that size would be appropriate to feed a subpanel (e.g small secondary consumer unit), and inside that, you could have regular breakers of appropriate size (13A or 16A) to feed to the heating tape. Every circuit thus fed would inherit the RCD protection from the pictured breaker.

It is normal in Europe to put the RCD on "the whole house" or a significant fraction of it. So 63A is a perfectly normal size for a 30ma rated RCD. The purpose of the RCD is reduce house fires by catching partial-short ground faults, and also providing limited protection to humans against severe shocks. This contrasts with American practice to put one 15-20A RCD (GFCI) per branch circuit, and the RCD is much more sensitive at 5 to 8 milliamps. The primary purpose of that RCD is to protect humans from shocks of modest severity. (A shock that stuns, also kills if it has knock-on effects of falling, drowning etc.)

So if you are powering a modest amount of heat tape (<2500 watts) and your primary purpose in RCD is due to an elevated risk of electrical shock, you may want to opt for a branch-circuit-sized 6ma RCD.

  • Hello, Thank you very much for your help. If I'm understanding it correctly, the 63A 30ma RCD is not sensitive enough to use for safety purpose, right? So I would need to get a 6ma RCD if I want to wire it as I described in the picture. I'm in the UK and the supplier that I'm looking at (RS component) doesn't sell 6ma RCD.. I was wondering RCDO would be a better and safer choice? such as 6 kA 25 A RCBO (with the trip sensitivity of 30mA)? Thank you. – Tquestions Sep 3 at 23:02
  • Yes, that would be better, but I am concerned that 25A is still a rather large breaker rating for a heating tape. Does the tape's documentation have anything to say about circuit breaker size? – Harper Sep 3 at 23:06
  • Generally, personnel protection RCDs in IECland are have IdeltaN ratings of 10mA (vs 6mA on UL943 Class A) – ThreePhaseEel Sep 3 at 23:17
  • unfortunately, the heat cable's manual does not have any information about the circuit breaker size. I found '1P 10 kA 10 A RCBO, Trip Sensitivity 30mA', would 10A small enough? I will also email the heat cable's supplier to ask if they have recommending circuit breaker size. – Tquestions Sep 3 at 23:23
  • @Tquestions can you get one with an IdeltaN (trip sensitivity) of 10mA vs 30mA? – ThreePhaseEel Sep 4 at 0:17

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