RCD

[Fingers]

This may be a stupid question.
If an inverter manual states it must be protected with a 40amp device, am I correct in thinking that this means the ampage should be the same 40amp rcd, and 40amp mcb?
I know it is I'm just having one of those dumb ass days.

I'm just thinking (over thinking), as a new consumer unit (breaker box) is almost always supplied with a 63amp rcd, for protection to all circuits. But how good can the rcd be if you use no where near that amount of amps? Is there a mathematical relationship between rcd and mcb where the rcd can be higher amps rating than the mcb?

 

[svetz]

If an inverter manual states it must be protected with a 40amp device...

Could just mean a 40 amp fuse (or breaker, or MCB), hard to tell without more context. Those devices protect the wire, not really the equipment.

An RCD (aka GFCI) is basically going to interrupt the circuit if the ampsIn ≠ ampsOut, a condition indicating current going somewhere unintended. Since they're safety devices, they generally trip in the milliamps and usually millseconds (although some can be fairly slow).

 

[SamDeleted]

RCD can be much bigger than 40amp , eg. 63amp, 80amp ,

Bigger is fine, smaller is not ok


The MCB needs to be correct as listed in the manual



Most people wouldn't bother with the RCD , but it doesn't hurt to have one, makes things safer. Rules and regulations are different country to country so might be required?

 

[SeaGal]

This may be a stupid question.

Questions can never be stupid... only the answers people give can be

If an inverter manual states it must be protected with a 40amp device, am I correct in thinking that this means the ampage should be the same 40amp rcd, and 40amp mcb?

It will refer to the cut-off trigger of an MCB (= fuse to us oldies!). Having said that, what size inverter are you considering and what size wire is connected to the inverter? The MCB must be rated less than the wire's current carrying capacity and ideally both should have a 25% margin on the current that will be carried.

An RCD, on the other hand, serves a completely different purpose as @svetz mentioned above. The RCB must be capable of handing the same or more current than the MCBs, but will trip when there is an imbalance between live and neutral or a leak to earth. The leakage current for a household RCD is 30mA.

It is not recommended to connect your solar system to the house side of the RCD. If you do so, you will likely get nuisance tripping as discussed on this thread...

New Solis inverter trips house electrics at morning start-up?

Hello there I have just installed a new Solis S6-GR1P3.6K-M-DC Inverter - this replaced a 10 year old+ Sungrow inverter that failed/out of warranty! None of the wiring set up has changed other than a new AC plug for the Solis inverter. I have a 16-panel 4kw system. It seems, when the inverter...

diysolarforum.com

 

[Fingers]

Thank you guys, every scrap of info helps

The MCB must be rated less than the wire's current carrying capacity and ideally both should have a 25% margin on the current that will be carried.

This got me thinking again about cables, as I did check before.
My Sunsynk inverters manual is strewn with typos, errors and contradictions all over the place.
A few days ago I noticed that the stated 32amp MCB should have been 40amp as stated somewhere else in the manual. The 6mm cable the manual recommends, will not be enough to take 40amps and have some safety margin.
Time to hunt for some 8mm

 

[SeaGal]

Thank you guys, every scrap of info helps

& gals

The MCB must be rated less than the wire's current carrying capacity and ideally both should have a 25% margin on the current that will be carried.

The 6mm cable the manual recommends, will not be enough to take 40amps and have some safety margin.
Time to hunt for some 8mm

What power is your inverter and how far will it be (cable length) from your CU? You need to consider voltage rise as well as current capacity.

Also factor in the routing of the cable (e.g. in open air / under loft insulation etc.) e..g see https://electrical.theiet.org/media...-ratings-for-cables-in-thermal-insulation.pdf

 

[Fingers]

It's a 5kw.
Sunsynk say it should have it's own breaker box for mains in/out and another for ups. Both units will be a couple of feet from the inverter.
Both will have Type B rcd, Type B mcb, spd with it's Type B mcb.

There's a 100 amp armored cable coming from the house main consumer unit via 65amp mcb, clipped to wall to the garage 8 meters away where it joins the inverter mini breaker box. And then I'll fit an 8mm2 cable to the inverter.
Another 8mm2 cable out to ups breaker box, from that another 100amp armored cable going from the garage clipped to wall to a mini consumer unit in the house. I plan to have this as a simple, 100amp switch, mcb's for socket, lighting and central heating, as the rcd and spd will still be active from the ups breaker box in the garage. But regulations will probably have me fitting another expensive type B rcd and spd with the lighting and socket mcb's.

The original house consumer unit will keep the high amps shower, and cooker cables (and a 6mm2 armored cable for back up grid tie in the garage. Just in case).

So, consumer unit, inverter breaker box, inverter, ups breaker box, consumer unit. Somewhere in all that I will have to fit the garage consumer unit. I think I'll take it from the ups breaker box.

I'll be glad when it's over.

 

[SeaGal]

All sounds good and sounds like you have plenty of capacity with your 100A SWA (=16mm2 I assume) for the upgrade to a 12kW inverter next year

 

[Fingers]

16mm2 I assume

Yes 16mm2.
I don't think I'll need anymore than the 5kw inverter.