Pure sine wave inverters. 12v /230v

[Scots John]

I routinely upgrade Motorhome systems. I do use Chinese made pure sine wave inverters from evilbay, usually 2500w/5000w (peak). I always sum the maximum off grid Continuous draw and double it or more. No problems so far, BUT I have heard of others Having problems.

i have pushed consumption higher than continuous max to check the thermal overload kicks in, and it does.

last point is when the Motorhome is hooked up to external mains, do you isolate your inverters from the circuit or leave them connected but not on? I have done this accidentally once and no I’ll effects, I just have limits how far I trust them.

 

[James]

It is not acceptable to have an inverter in parallel with a grid supply unless it is suitable for grid tied operation.

if you want to use a cheep inverter, the right way to do it would be to install a changeover switch that can select one of the three (grid-off-inverter)

there are some combined charger/inverter units available that ARE designed to do this and they will charge the batteries when grid power is available and then feed the 230v equipment from batteries when grid is off.

you are a new member so i have no feel for your electrical knowledge yet,
Not intending to insult you but, do you have any electrical qualifications?

 

[Scots John]

It is not acceptable to have an inverter in parallel with a grid supply unless it is suitable for grid tied operation.

if you want to use a cheep inverter, the right way to do it would be to install a changeover switch that can select one of the three (grid-off-inverter)

there are some combined charger/inverter units available that ARE designed to do this and they will charge the batteries when grid power is available and then feed the 230v equipment from batteries when grid is off.

you are a new member so i have no feel for your electrical knowledge yet,
Not intending to insult you but, do you have any electrical qualifications?

Not insulting at all. Retired firefighter, extensive home workshop facilities from a lifetimes tinkering. Building, electrical, mechanical and some engineering on my lathes.
I do have an isolating switch between my inverter set up (runs from leisure batteries). When on mains hook up I always normally isolate the inverter except one occasion I forgot, silly mistake, but no damage.

motorhomes are equipped with Zig charging system that automatically charge the leisure batteries first to full capacity, and then the vehicle battery. There is therefore no need for a combined unit.

 

[James]

the key is that the inverter must be isolated from the mains (grid) supply at all times, otherwise there is a possibility of back feeding into the grid. This is absolutely forbidden so as to avoid killing the engineers that are working on it when the power fails.

automatic grid-inverter-charging systems are available and are equipped to charge both leisure and engine batteries.
they are often specified for high value floating gin palaces and other yachts.
for a motor home i would be looking at an isolator that sits between the mains input socket, the inverter and the distribution board.

one thing that is worth noting is that with many of the cheaper inverters, they have poor isolation between the 12v side and the 230v side of the transistors.
the 12v side has a connection to the vehicle chassis (-ve) under some circumstances it is possible to get a shock from the vehicle ground when you are standing on real ground (terra firma)

dont ask me how, but i know when i fitted a 1kw inverter to my car many years ago, it had some strange side effects that meant that i removed it for safety reasons.

 

[Scots John]

the key is that the inverter must be isolated from the mains (grid) supply at all times, otherwise there is a possibility of back feeding into the grid. This is absolutely forbidden so as to avoid killing the engineers that are working on it when the power fails.

automatic grid-inverter-charging systems are available and are equipped to charge both leisure and engine batteries.
they are often specified for high value floating gin palaces and other yachts.
for a motor home i would be looking at an isolator that sits between the mains input socket, the inverter and the distribution board.

one thing that is worth noting is that with many of the cheaper inverters, they have poor isolation between the 12v side and the 230v side of the transistors.
the 12v side has a connection to the vehicle chassis (-ve) under some circumstances it is possible to get a shock from the vehicle ground when you are standing on real ground (terra firma)

dont ask me how, but i know when i fitted a 1kw inverter to my car many years ago, it had some strange side effects that meant that i removed it for safety reasons.

I actually feed the inverter supply in through a switched mains plug which I also unplug except on that one occasion. I do appreciate what you are saying about back feeding.

I haven’t experienced any unusual issues, but when it’s not in use for more than a couple of days I disconnect the battery leads too as there is always a small drain value.

this is the one I usually buy.

https://www.ebay.co.uk/itm/203050794251

 

[James]

am i understanding correctly?
the inverter output is wired to a plug that you insert into one of the sockets in the motorhome to provide power to everything else??

 

[Scots John]

am i understanding correctly?
the inverter output is wired to a plug that you insert into one of the sockets in the motorhome to provide power to everything else??

Yes. It can be directly connected/disconnected. There are only four outlets in a motor home.

this way when on hook up, the plug becomes an outlet.

i understand your concern, I have previously connected via an illuminated switch, but I prefer to visibly see it is not connected. Except as I said on the one occasion I forgot. The inverter only powers the microwave and tv, and occasionally a hair dryer etc.

 

[DPG]

Please use a different solution to supply power!! Your method is liable to kill someone. A plug used to supply power is known in the trade as a widow maker.

Consider a commando style mains inlet.

 

[Scots John]

Please use a different solution to supply power!! Your method is liable to kill someone. A plug used to supply power is known in the trade as a widow maker.

Consider a commando style mains inlet.

The plug cannot be live unless the inverter is connected and switched on!

the plug is ALWAYS connected before the inverter is switched on. I’ve been using this system, as do others i know, for years. The socket is also switched.

a commando inlet inside A Motorhome? Inverters have to be sited close to the battery and using heavy leads suitable for the intended load. I put them near a floor vent on aluminium box frame under a seat. A smoke detector is also fitted under the seat as well as in the living area. The batteries are vented through the floor by tubes.

i agree a more suitable connection would be great, but not something like the illuminated switch I used to use, because I want to see it is visibly disconnected (at the same time as its isolated from the batteries).

i Could connect it permanently and use the plug on the inverter to isolate at the same time as isolating the battery supply, but then it would be out of sight. An ideal solution would by a power source selector switch at the hook up inlet cover, but there simply isn’t room For a two pole changeover switch there.

 

[James]

You have had several solutions suggested to you.
I have no intention of starting a long running argument here.

Consider this
One day whilst you are enjoying an evening with friends outside
There child who has been sitting quietly all evening playing on an iPad, pops inside for a moment.
When they pull out the plug to plug in there iPad charger

They come into contact with the live pins on the plug .
All you are likely to hear is a small thud as they drop dead to the floor.

Worth noting that there is nothing in that circuit that will switch off the power to the lethal plug. No rcd protection and a fuse is not going to blow in the plug either.

If you can’t grasp the fundamental dangers involved in this arrangement then I suggest you leave electrical work to others who do.

 

[Scots John]

You have had several solutions suggested to you.
I have no intention of starting a long running argument here.

Consider this
One day whilst you are enjoying an evening with friends outside
There child who has been sitting quietly all evening playing on an iPad, pops inside for a moment.
When they pull out the plug to plug in there iPad charger

They come into contact with the live pins on the plug .
All you are likely to hear is a small thud as they drop dead to the floor.

Worth noting that there is nothing in that circuit that will switch off the power to the lethal plug. No rcd protection and a fuse is not going to blow in the plug either.

If you can’t grasp the fundamental dangers involved in this arrangement then I suggest you leave electrical work to others who do.

I do understand what you are saying. The socket is deliberateLy sited where I can get at it, not a child.

I don’t like doing this, I just haven’t found an alternative solution, so I take all possible precautions to prevent accidental access to this plug Including a warning label.

you see the set up inside, out is a locker with the hook up inlet with no space for any switchgear. how would you approach this?

 

[James]

https://uk.rs-online.com/web/p/non-fused-switch-disconnectors/8609554/

 

[Scots John]

https://uk.rs-online.com/web/p/non-fused-switch-disconnectors/8609554/

I have already looked at these, but where to site a 7” by 6” box? There are IP67 boxes on eBay as I said I would seriously like this option, but I was looking for interior alternative like a two pole fish key switch or similar that was rated for the two voltages.

most are as this rated for much higher voltages.

 

[DPG]

Remove the plug and advise the other people who are using the same method to do the same. It is dangerous.

Also bear in mind what may happen when one of these is sold on again to someone who may not be as aware of how dangerous it is.

 

[Scots John]

Remove the plug and advise the other people who are using the same method to do the same. It is dangerous.

Also bear in mind what may happen when one of these is sold on again to someone who may not be as aware of how dangerous it is.

I have always explained the system thoroughly when I’ve sold one. the first one I bought years ago had this system in it too.

however, I take your comments on board.

i located a 10 amp fish key switch but it’s an inductive load switch for motors etc, do you know of any for domestic applications?

 

[James]

You need to recall all the units you have sold.
Instructions supplied or not, what you are selling is a device with a potentially fatal design flaw.

A judge is likely to give a custodial sentence to you when someone is killed by one.

 

[pc1966]

The answer is, and always should have been, a change-over switch so you have a male socket for feeding power in (i.e. safe to touch female plug on the inverter output) that can never be live as is is only connected when the mains supply is disconnected. Or have the inverter hard-wired to the change-over switch if the inverter is not frequently removed.

If you only need 16A or less you can use the IEC style C19/C20 that you see on the likes of 3kVA UPS, they are muhc smaller than commando style connectors (but not as strong, watrerproof, reliable, etc, so pay your money and take your choice).

It is absolutely insane to have a live plug, further to have the potential to be in parallel with the AC supply without means of isolating/shutting down on supply failure to protect utility workers from back-feed!

 

[Lucien Nunes]

What needs to be said has been said but I would just like to add my voice. Configuring the plns of a plug to be live from the inverter is a serious violation of all normal safety standards, non-compliant, probably uninsurable and frankly irresponsible. The same is true of connecting an inverter output to a circuit that can be simultaneously energised from the mains. Units so equipped need to be recalled and rectified.

On a purely electrical note, the outputs of normal (non grid-tied) inverters are not capable of withstanding the kind of transients that can occur on the mains. One spike and the transistors in the output H-bridge will shoot through and be blown to smithereens. If the transformer insulation or RF bypass capacitors fail, the mains will be connected to the 12V circuit.

 

[davesparks]

The plug cannot be live unless the inverter is connected and switched on!

In other words the plug will be live if the inverter is connected and switched on.

This arrangement is commonly known as a widow-maker.

 

[Lucien Nunes]

It's the most ludicrous proposal I had seen in a long time which I suspected to be trolling and not a real thing. After a while it started to nag me that he might actually be doing this for real, so I replied. I wonder how many people have had shocks since the thread was current 2 years ago.

Hi @HowelGrace, welcome to the forum. I get that your post is ironic, but I'm not sure what the OP would make of it? I worry that it might be taken as a compliment! Being over here in the UK I'm a bit rusty on the NEC, can you remind me which code chapters deal with back-feeding of panels and interlocks?

 

[plugsandsparks]

Quite frankly its a bizarre solution for a problem that i solved over 30 years ago. I had Motorhome, needed 240V for phone chargers, nice TV, sat receiver and playstation, so wired an inverter, as you do, BUT it was terminated to a dedicated double socket where all the above were located and next to a double socket which would be powered under hook up conditions. All i needed to do was unplug the stuff from inverter socket to hook up socket. Terminating on a plug and then backfeeding the whole electrical installation was simply not necessary so i do not understand what problem is trying to be solved here.

 

[Lucien Nunes]

If the OP was serious, neither do I. Or rather, it's a common problem with a very simple solution - a 3-position DP transfer switch. The OP claimed there was no room for the switch and I appreciate that not every motorhome has a conveniently located engine room like my boat (where my ATS is). OTOH I presume there was still some space at the morgue.