To Trip, or not to Trip

[Willum]

Our faithful old Frankia i7000BK, recently much discussed ref EBL/Fridge, has thrown up another question.

(No face-palm interface, or fits of laughter, Keithl, I can hear the collective groan from here! (lol) )

 

It has had a mains/inverter changeover relay fitted which functions OK and appears - even if not beautifully done - to be quite safely connected and protected - that is 'protected' in the mechanical sense. It is wired to ensure only a few of the sockets are powered up by the inverter, not the fridge/immersion/hab battery charger, etc.

The changeover relay is wired to bring the 240volt mains on line to all consumers when it detects the EHU has gone live and is fitted after the incoming RCD/trip enclosure, so none of the circuitry supplied by the inverter is protected by the RCD/trips.

I should add that it's a full sine wave 600 watt inverter, so no heavy (battery-killing) use and is properly fused internally and externally on the supply side.

I see many and varied theories regarding the safety/practicality side of things but does anyone know of an accepted, safe route that doesn't involve just ripping it out.

In short, it works, I like it - but could it be better protected?

.......... and, for the cautious among us, it will all get a full blown safety check of the installation before it goes into service, whatever happens.

 

Will

[colin]

I've only had a quick think about this, but it strikes me that you will be pushing the EBL charger to, or beyond it's safe limits when using those sockets if not careful.

[Willum]

colin - 2021-01-27 12:41 PM

 

I've only had a quick think about this, but it strikes me that you will be pushing the EBL charger to, or beyond it's safe limits when using those sockets if not careful.

Errrmmmm!

Charger is never in circuit when the EHU isn't live from what I can see I wrote, Colin.

"It is wired to ensure only a few of the sockets are powered up by the inverter, not the fridge/immersion/hab battery charger, etc."

 

Will

[colin]

My mistake, only had a quick glance at post before going out to do a job.

So if I've got this right, you are concerned that when not on mains, and the invertor is in use, there is no RCD protection?

Just read again, and it seems if the EHU RCD/MCB trips, then the invertor would cut in, that doesn't sound good.

 

[Willum]

colin - 2021-01-27 1:23 PM

 

My mistake, only had a quick glance at post before going out to do a job.

So if I've got this right, you are concerned that when not on mains, and the invertor is in use, there is no RCD protection?

Yes, exactly that. I've seen arguments both ways regarding RCD protection, just looking for reasons/arguments.

.......... and my turn to have missed something. It isn't idiot proof but the system would require the inverter to be deliberately left live for it to take over - and ours now has adequate visual warning of being 'ON' and a buzzer if it overloads - but it only powers a few sockets.

 

Will

[mikefitz]

comments.

 

To fully comply with regulations the cable between the input socket to the 'consumer unit' must be a continuous 3 core flexible multi strand 2.5mm2, no longer than 2 meters well supported and protected against damage. The feed/s to the 230v sockets and appliances must be via a RCD and MCBs, these protective units must break both live and neutral with a fault or overload. The incoming protective earth wire must be connected to the chassis /body.

 

Inverters may be considered as 'portable' or 'permanent'. With a 'portable' system and a single appliance connected via its built in socket, the system is moderately safe provided the leads and appliance are not damaged. It could still kill you by simultaneously touching the two live wires.

Where its a 'permanent 'installation, with extra wiring, multiple sockets, and possibly supplying multiple appliances, the risk increases. To protect against this increased risk a RCD must be fitted at/near the inverter output, AND the inverter must be a type with an internal neutral to earth wire connection or be modified as indicated in the user instructions. Unless the inverter is supplied with the neutral to earth bond or has the option, it may be unsuitable for this application, as without the neutral bond and this protective earth connected to the van body/chassis, the RCD will not work.

 

The inverter neutral to earth bond must only apply the the inverter powered circuit and must be totally separate from the incoming 230v from the hook up point, thus any switching circuit, if fitted, must take this into account by switching both live and neutral poles.

 

I consider any auto switching of inverter power using a DIY approach, a safety issue, especially with no RCD protection in circuit. Its all too easy for a 'dead' system to become 'live'.

 

Mike

[Willum]

Thanks for the response.

 

I fully understand your misgivings and it's why I asked the question.

This is the way I found it and I would prefer the option of using the setup - with the proviso that it's safe, not just for me who knows about it but anyone who doesn't, as well. If feasible it will get whatever modification it needs, then tested by our local electrician - or it will get binned. (I own and use a fully functional, recently certificated, Metrel MI3000 and will be testing the RCD regardless.)

 

So far:

 

Cable from input to Consumer Unit: complies - easily

 

Cables from CU to all appliances: ignoring the presence of the inverter, they are all routed through a (2 pole) RCD and 2xMCBs, so both live and neutral broken.

 

Incoming supply earth (is it a CPC in this instance before the CU?) is bonded to the chassis from the CU with 6mm2

 

Inverter RCD/MCB protected? No, not when it is 'on line'. I'll recheck the docs for detail but I doubt it will have anything internal except its two blade fuses, which I would think are purely supply side related. The changeover relay is post CU - and what I didn't like to begin with.

 

In reference to the inverter itself:

I have the manual and it has references to the earth bonding side of it. I'll read carefully about the neutral/earth connection. I've a memory it's an external link to be made but will check it's right.

With regard to the auto switching of EHU/inverter power: before I asked the question I thought that one major thing to be done was the fitting of a suitably rated and protected 2/3 pole ON/OFF/ON manual changeover switch before the CU, not just rely on an NC/NO relay.

That would surely confirm the isolation of one circuit from the other?

Doubtless no need to tell you - and I've got recent experience - that relays can and do fail, usually in the mode that you don't want.

 

Will

[Willum]

Update:

The neutral/earth link for the inverter is achieved by a simple switch on the exterior.

The manual also quotes it as being RCD compatible.

 

Will

[mikefitz]

Hi Will, with your test meter you should not need independent assessment of the system.

 

I agree with the use of a three way center off switch as a foolproof method of switching the inverter, suitable switches are available,

 

https://uk.rs-online.com/web/p/non-fused-switch-disconnectors/1038454/

 

It sounds very much that the inverter can be neutral to earth bonded so is usable. A suitable RCD with enclosure on the feed between the inverter and three way switch is needed. No need of a MCB since the inverter could never generate enough power to trip the unit, but useful as an isolator or to fill the space in the enclosure.

 

Mike

[Willum]

So far, so good. Thanks. It's looking as if it's safely achievable. There must be thousands of motorhomes with an inverter fitted that just are not properly protected. I've certainly owned one, I don't want to own another.

 

I now have to work a connection method that ensures the 'unwanted' consumers, ie, fridge, immersion, charger, etc, don't come on line with the inverter but the few sockets I'd like to be able to use are available.

 

Will

[flyboyprowler]

747Heavy - 2021-01-29 7:44 AM

 

So far, so good. Thanks. It's looking as if it's safely achievable. There must be thousands of motorhomes with an inverter fitted that just are not properly protected. I've certainly owned one, I don't want to own another.

 

I now have to work a connection method that ensures the 'unwanted' consumers, ie, fridge, immersion, charger, etc, don't come on line with the inverter but the few sockets I'd like to be able to use are available.

 

Will

Not on your topic I know, but are you an ex 747 (or similar) pilot?

[Willum]

flyboyprowler - 2021-01-29 1:05 PM

 

747Heavy - 2021-01-29 7:44 AM

 

So far, so good...........

Will

Not on your topic I know, but are you an ex 747 (or similar) pilot?

No, 35+ years as a Maritime Military and Government contract flyer, mainly as an Air Electronics Operator/Systems Operator.

I was taught to fly twin turboprop, IMC as well, as an 'Oh, crap!' backup in case of a bad birdstrike as we spent most of out time below 1000ft over the ocean.

The 747Heavy came from my old Burstners A747 and 748, GVW taken up to 5750Kg. There was also a very strong nod in the direction of a sadly recently passed away close friend and working colleague (RADAR-based) who was for many years a 747 pilot and a 757 Captain.

There yer go. Short version bio :-D

....... and the 'Prowler'? I certainly know about them in one aspect of flying not a million miles adrift from my own! :D

 

Will

[flyboyprowler]

747Heavy - 2021-01-29 2:20 PM

 

flyboyprowler - 2021-01-29 1:05 PM

 

747Heavy - 2021-01-29 7:44 AM

 

So far, so good...........

Will

Not on your topic I know, but are you an ex 747 (or similar) pilot?

No, 35+ years as a Maritime Military and Government contract flyer, mainly as an Air Electronics Operator/Systems Operator.

I was taught to fly twin turboprop, IMC as well, as an 'Oh, crap!' backup in case of a bad birdstrike as we spent most of out time below 1000ft over the ocean.

The 747Heavy came from my old Burstners A747 and 748, GVW taken up to 5750Kg. There was also a very strong nod in the direction of a sadly recently passed away close friend and working colleague (RADAR-based) who was for many years a 747 pilot and a 757 Captain.

There yer go. Short version bio :-D

....... and the 'Prowler'? I certainly know about them in one aspect of flying not a million miles adrift from my own! :D

 

Will

34 years in civil aviation plus instructing before, and the Prowler was made up one dark lonely night en route to Palma!! Started on 748's then 727, and finished on 737. No long haul which is where you always heard the "heavy" as they trundled off across the pond.

[Willum]

flyboyprowler - 2021-01-29 2:51 PM

 

747Heavy - 2021-01-29 2:20 PM

 

flyboyprowler - 2021-01-29 1:05 PM

 

747Heavy - 2021-01-29 7:44 AM

 

So far, so good...........

Will

Not on your topic I know, but are you an ex 747 (or similar) pilot?

No, 35+ years as a Maritime Military and Government contract flyer, mainly as an Air Electronics Operator/Systems Operator.

I was taught to fly twin turboprop, IMC as well, as an 'Oh, crap!' backup in case of a bad birdstrike as we spent most of out time below 1000ft over the ocean.

The 747Heavy came from my old Burstners A747 and 748, GVW taken up to 5750Kg. There was also a very strong nod in the direction of a sadly recently passed away close friend and working colleague (RADAR-based) who was for many years a 747 pilot and a 757 Captain.

There yer go. Short version bio :-D

....... and the 'Prowler'? I certainly know about them in one aspect of flying not a million miles adrift from my own! :D

 

Will

........

No long haul which is where you always heard the "heavy" as they trundled off across the pond.

Exactly that - T/O weight above 130-odd tons from memory?

Burstner wasn't - but felt like it when you wanted to stop in a hurry...... 8-)

 

Will

[Willum]

Right! Back into the fray:

 

I've been mentally running through the fault scenarios that I can think of. If anyone can see a potential hazard, I'd be grateful for their input.

 

The result of my calculation of the (hopefully much safer) system is that:

the 3 pole, 3 way (ON-OFF-ON) switch is going to be placed after the two inputs (EHU and inverter) into the system and used to couple the appropriate supply on to the selected outputs. Both of these supplies are independently grounded to the vehicle chassis and will have their own individual (30mA) RCD and MCBs, so any of the normal overload/short faults are catered for.

The heavy consumers (fridge, hab battery charger, immersion heater, microwave) will only come into circuit when a relay is energised by the EHU input and it swaps from 'normally open' to closed and connects them.

 

When plugged in, the EHU connects directly to the heavy consumers and to the socket spurs because it closes the relay contacts to connect them as well.

If the relay fails to operate and remains 'normally open', the EHU won't come back on to the socket spurs. If it fails closed (eg, welds shut under load) it is no problem, the system is doing it's normal (pre-inverter installation) job if the EHU is connected.

If the foregoing (relay welded shut) happens undetected and the inverter is brought back on, it will simply trip out/pop its supply fuse/MCB because of the (potential) extra load.

Either way, the inverter is off line 'til the problem is solved.

The inverter cannot feed power back to the EHU end because of the 3 way switch.

Likewise, if the EHU is brought on line with the inverter still switched on , it cannot destroy the inverter because the 3 way switch stops the connection being made.

 

A fault of any sort detected by either of the respective supply RCDs will knock its associated supply off line. If the fault is on a consumer, it won't be detected by the inverter RCD but even if the supply source is swapped back to EHU, that 'new' supply will detect the same fault and trip.

 

One thing I did note is that it appears Mr Frankia didn't anticipate a particularly heavy load on the system, full stop. The original and still present MCB is only 10 amp. The fridge has its own of the same value, wired in series. I guess that's probably a function of no electric heating of any great value, eg Alde, just 450 watts of Truma immersion heater.

 

My brain has filled up, so any arguments/missing bits, please say ............ and I have a question for the more knowledgeable:

 

Do I switch the earth (CPC) conductor (not the EHU/Inverter grounding lines) through the 3 pole, 3 way switch, along with the live and neutral, or do I just connect every earth together?

From what I can see, if an earth fault develops, it wouldn't matter if both RCD's popped. The fault would have to be removed before anything went live again, regardless of which supply saw it?

 

Will

[Willum]

This is what I've come up with and it seems to work well. I've tried lots of scenarios and the 3 way switch is the key to keeping it all separate. In the end I wired all the earths to the same point and didn't switch them. If a fault is detected, I suspect it will probably take both RCDs off until the fault is removed. Not a bad idea to my way of thinking.

The 4 pole relay is a substantial 25 amp Schneider unit, as is all the other new enclosure and MCBs. Apart from a soft 'thunk', there's no noise from it at any stage.