RCD design - Wiring unlimited rev 01 - 03/2023

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arildo asked Jan 2, '24

RCD design - Wiring unlimited rev 01 - 03/2023

The Wiring Unlimited is a valuable document that deserves to be maintained and developed. I hope to understand parts of it better and possibly help to improve. Throughout Chapter 7 there are several considerations about RCDs that I am getting thoughtful over.

Overall the message in the document seem to be that RCDs are useless unless the neutral is grounded. This should in fact mean that RCDs are useless in IT-networks.

Am I right to say that RCDs are not useless in IT networks? In fact I claim they are even more important than in TN systems. RCDs in IT networks provides early detection of second earth fault and more importantly may save a life if the system already contains one fault and a person introduces the second.

There is also one paragraph on page 62 that says that pressing the test button of an RCD is useless if the neutral-to-earth link is missing and that you will get a false impression that the RCD is operational. I would disagree. It was explained just a few paragraphs above how the test button works and this has nothing to do with the earth link. Then the main point is that the RCD will work as long as e.g. 30mA or more is leaking from the circuit.

In installations with a lot of components and numerous terminations, it is difficult to speculate in the likelihood of multiple semi-earth faults creating leakage currents. There is also a capacitance between earth and the live parts in IT networks that serves as a path for the first fault current. This current may be small in compact off grid networks, but may get significant enough in larger networks to enable the RCD to trip on the first fault in IT networks.

I would very much like to see Wiring unlimited to be more precise on this topic and make it more clear what earthing system a paragraph relates to. Also I would like to see a full drawing of an IT system together with the other drawings on page 64.

In an off grid network, is there really any reason why you should not use IT network?

I wonder if there are other perspectives on this topic? Please feel free to elaborate.

system design Grounding rcd tripping

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kevgermany ♦♦ commented · Jan 02 at 08:56 PM

I think this is for Victron to address, not the community.

@Guy Stewart (Victron Community Manager)

Looks like one for you.

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arildo kevgermany ♦♦ commented · Jan 02 at 11:03 PM

Thanks for the comment and the forward. I guess my topic is both for discussion and at the same time feedback to Victron.

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kevgermany ♦♦ arildo commented · Jan 03 at 05:19 AM

They are always very grateful for constructive feedback and discussion.

Might take a while to get a response at this time of year, though.

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Mike Dorsett commented · Jan 06 at 09:12 PM

Specs for an IT system require a hihger level of insulation, and a higher level of maintenance than a TN system. You also need equipment to detect the first fault, and it is then Mandatory to clear this. So while an IT system may look attractive to an off grid system, a TN system is easier and safer to implement, as the RCD's will work normally -on the first fault.

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arildo Mike Dorsett commented · Jan 08 at 03:20 PM

Hi Mike,

Thanks for your comment. Could you say more about why you need higher level of insulation? The line to line voltage is the same and the neutral conductor needs to be able to carry the same current as the phase conductor, as long as it is a single phase system

A good point about the insulation monitoring, but what if the monitoring system detects leakage current? Then what is the point of having both RCD and monitoring? I beleieve this is the reason why RCDs have replaced monitoring in IT-nets. Accidents have happened because the monitoring was giving an alarm, but did not really give any protection until the fault was cleared. Potentially dangerous if the personnel around does not understand the alarm and what to do.

In any case, most earth faults are not with zero resistance, so I don't agree that all earth faults in TN net will trip the MCB. High resistant fault current may be smaller than the trip current no matter the system, TN or IT. The difference being that in IT net, a low resistance fault will only create a small fault current through capacitance unless there are other existing faults in the system. Which in time there will be a number of high resistance faults on both phases, each contributing with a very small leakage current.

If you wish to clear a fault with less than 30mA fault current, then you can of course use monitoring to detect such small currents, but they may not be from a fault. They could just be capacitance leakage and this you cannot clear.

I am not speaking against TN. I am just curious want to check my own thoughts on this.

I think TN is not always easy. Specially if you have more than one source of power. then it becomes important to maintain only one system reference conductor at a time, between earth and neutral conductor. This is the reason for the relay inside the Multiplus that makes and breaks between earth and neutral. I find it sometimes challenging to find documentation and assess what is going on inside generators and invertors with regards to the connection between neutral and earth. This of course being information needed for any kind of system. E.g. you don't want to set up an IT system if your generator is internally wired with one pole connected to earth, and vice versa .

Glad to hear your thoughts.

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