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binky

Unbalanced loads on a 3 phase RCD

Question

binky

I've been studying some diagrams of 3 phase RCDs, as far as I can see, because all 3 phases pass through the same detection coil, then any imbalances on the phases doesn't affect the RCDs' operation?

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Andy™

yep. only an imbalance that misses the RCD, like to earth

 

if it didnt, id like to see you balance all 3 phases to within 30mA of each other

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Evans Electric

I'd say thats been answered.

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binky

Just wanted to check. I've been looking at a caravan site that has a pretty ropey set of TT electrics. They want solar panels, but there isn't enough space for a 3 phase system. A small single phase system is unlikely to back-feed much / anything to the grid, but the last thing I need is a system that starts tripping the incoming 100mA Type S 3 phase RCD.

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Geoff1946

I would run that idea past the supply company before committing any expenditure. They may not be too keen on a micro-gen system that unbalances the supply.

 

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Tony S

A single phase input to the local three phase grid shouldn’t be a problem. It’s no different to a house with a single phase supply which has a PV array feeding in to the grid.

 

If you start pumping 10’s of kW in to the local system then it could get hiccups but from what you say that shouldn’t happen.

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binky
10 hours ago, Geoff1946 said:

I would run that idea past the supply company before committing any expenditure. They may not be too keen on a micro-gen system that unbalances the supply.

 

not an issue, can't get a system big enough on this site to cause problems like that.

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Sidewinder

It would be disconnection times I'd be worried about, feeding in solar, 5s disconnect time, downstream of an RCD which needs to disconnect in a lot less than that, unless you can guarantee that the solar is truly and totally unreferenced to true earth.

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binky

the RCD in question is only protecting sub-mains and could be replaced with 300 or 500mA unit. 30mA protection is provided where it is needed to protect people.

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Sidewinder

That still doesn't matter though.

If the solar has an earth reference to the inverter output, i.e. if in the event of the upstream device disconnecting, current can flow from the live of the inverter output to earth, then the disconnection time will be that of the solar inverter.

Which iirc according to product standards is 5s.

Whereas the max disconnection time on a TT is 1s.

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binky

contravening info, 1 second for ccts / distribution ccts over 32A, but reading GN1, section on RCDs 3.7.7 in my copy, it talks about the actual requirement of TT systems is to restricting touch voltages to 50V. My older version of GN8 (2004) also describes this as being the actual requirement for TT systems and a disconnect time of 5 seconds as inferred from the appropriate equation.

Edited by binky

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Sidewinder

Well BS7671 says what is in the attached pdf.

 

TT Disconnection Times.pdf

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binky

I don't have my big yellow book to hand, but that table refers to circuits protected by an OCCPD, not RCDs

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Andy™

an inverter must not share an RCd with anything else. if someone comes into contact with a live wire and the RCD trips within 0.2 seconds, the inverter could still be supplying power for a further 5 seconds

 

also, if any device is providing ADS for any circuits then the inverter must be connected to the supply side (712.411.3.2.11)

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Sidewinder
33 minutes ago, binky said:

I don't have my big yellow book to hand, but that table refers to circuits protected by an OCCPD, not RCDs

 

No it relates to disconnection times.

Nothing to do with how disconnection is achieved, just that it MUST be achieved in those times.

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binky
24 minutes ago, Andy™ said:

an inverter must not share an RCd with anything else. if someone comes into contact with a live wire and the RCD trips within 0.2 seconds, the inverter could still be supplying power for a further 5 seconds

 

also, if any device is providing ADS for any circuits then the inverter must be connected to the supply side (712.411.3.2.11)

 

I'm well aware of that Andy, but any circuit anyone is likely to get belted from has 30mA protection at sub-boards. The Sub- ain in question os buried from the incoming point to a distribution cupboard in the centre of the caravan site

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binky
4 minutes ago, Sidewinder said:

 

No it relates to disconnection times.

Nothing to do with how disconnection is achieved, just that it MUST be achieved in those times.

 I've been reading the GN books, now mine are a little out of date, ie 2004 and 2008. The earlier version states quite categorially that disconnection time is not the main criteria for TT when disconnection times cannot be met and RCD protection is introduced to compensate and limit touch voltages to <50V , by reference to a formula, which implies a disconnection time of 5 seconds is acceptable. This advice seems to be getting watered down over time.

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Rob_the_rich

Would another 100mA s-type RCD installed at the output of the inverter provide the required disconnection time?

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steptoe
1 hour ago, binky said:

 I've been reading the GN books, now mine are a little out of date, ie 2004 and 2008. The earlier version states quite categorially that disconnection time is not the main criteria for TT when disconnection times cannot be met and RCD protection is introduced to compensate and limit touch voltages to <50V , by reference to a formula, which implies a disconnection time of 5 seconds is acceptable. This advice seems to be getting watered down over time.

From my understanding the <50v touch has been superseded by time if disconnection, not purely limited by the voltage rise.

Which, IMHO, can only be a good thing, after all, 49v at 100amps is still one helluva kick for the next 4.9seconds.!

 

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Sidewinder

I can't put quotes from the regs straight into the forum because the formatting is lost and it looks nonsense.

 

Attached, the relevant regs.

TT, disconnection times, must be met, 411.5.3.1.

Chapter 41 Protection Against Electric Shock Excerpt 1.pdf

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binky

OK, so both criteria need to be applied.

 

I've been checking the G83 and G59 settings for SMA inverters loss of mains test. The standards have been altered, trip times are now 2.5s max, however SMA set their inverters for 1 second and according to the test sheets actually trip in 0.3seconds. So the answer is chose the right inverter.

 

One thing that has occurred to me, if I rodded the inverter and do not connect the AC side earth to the shared earthing for the system how does that affect the requirements of the above?

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Sidewinder

I believe you must rely on specifications, not actual readings for your values.

 

You will still have an earth path will you not because both the ac side of the inverter and the installation will be connected to earth, i.e. true earth.

 

Is the SMA suitable for TT or only TN?

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binky

Never seen an inverter specced as being for TT only, they can generally be used for any earthing system. About all the manuals say is 'install to local regs'.... Must be loads of 16th edition TT installations that are incorrect, but in  most domestic settings it isn't hard to get access to the main incoming point.

 

This site has an earth rod at the distribution boards located about 150m from the main incoming point, I've not checked to see if it connected to the earthing for the sub-main cable, I suspect it probably is, not that that would stop a back-feed to the main incomer should that cable get damaged.

 

I've attached a cert for a Samil inverter, seems these are set for 0.5s. I get your point about standards, but if I can demonstrate that a particular brand of inverter complies with requirements of BS7671, I don't see a problem, would need to mark up installation that this is the case / cert accordingly.

SolarRiver 1100TL-S-2600TL-S-Verification test report-G83-2 for UK_EN.pdf

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Sidewinder

I don't think you will find an inverter that is TT only, but you will find them that are TN only.

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Rob_the_rich

Surely if the inverter is referenced to true earth, then an s-type 100mA rcd on the output of the inverter will trip within 500ms, about the same time as the s-type 100mA RCD on the mains side tripping, from the same fault. If both RCDs trip according to standards, disconnection of both supplies should occur in less than a second. Or am I missing something?

 

 

 

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binky

not necessarily as the RCd may not see a supply fault. I prefer to avoid RCDs when installing inverters, the relays tend to chatter on start-up and shut down which has a habit of upsetting RCDs. They are usually OK with a dedicated RCD, but any RCD shared is a mistake.

Edited by binky

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