Do I need a RCD on an inverter? (1 Viewer)

[RogerThat]

Hi everyone,

I'm debating buying an inverter but I'm a bit unsure about the RCD situation.

My leisure battery is mounted under the drivers seat and if I go ahead I would bolt the inverter directly on to the rear of the drivers seat base frame. This means the wires to the battery would be really short, maybe 10" or less (not sure that has any bearing on anything?).

I had thought that I would just use an extension lead from the inverter directly to the appliance in question.

So for example, if I wanted to plug a desk-type fan in while wild camping, I'd run a normal household mains extension lead from the inverter to wherever I decided the fan needed to be (somewhere in the rear lounge probably).

So apart from the obvious trip hazard of a trailing mains cable running from the cab to the rear lounge for example, would I also need to fit an RCD somewhere? Or would whatever fused protection the inverter offered be sufficient?

I haven't yet decided on a size, make or model of inverter (suggestions welcomed by the way!), I'm just trying to assess the overall cost and any unforeseen issues that I might not have considered at this stage

 

[maxi77]

Cheaper from CPC

http://cpc.farnell.com/pro-elec/kp-b13c/rcd-in-line-rewireable/dp/PL15119

 

[Techno]

Cheaper from CPC

http://cpc.farnell.com/pro-elec/kp-b13c/rcd-in-line-rewireable/dp/PL15119

That is too slow it needs to be 30 ma 30 ms

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[hilldweller]

The real question is..............

Do inverters have a N-E bond or is the 240V just floating ?

 

[scotjimland]

An RCD is not required.. it would only work if the Neutral at the inverter were grounded..

 

[Techno]

It varies but needs to be neutral earth bonded for an RCD
Sterling pure sine units can be altered for either

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[Techno]

Easy to check, if the test button does not trip the device then it is a centre tapped inverter

 

[Techno]

Mine was neutral earth out of the box as I believe most larger inverters will be

 

[RogerThat]

I'm still confused

Do I need a RCD?

I don't intend running the extension lead outdoors and I don't use RCDs on extension leads that I have in my house.

But maybe that's the point? I have a fusebox on the house, I don't have with an inverter?

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[Techno]

I'm still confused

Do I need a RCD?

I don't intend running the extension lead outdoors and I don't use RCDs on extension leads that I have in my house.

But maybe that's the point? I have a fusebox on the house, I don't have with an inverter?

Read the inverter instruction book

 

[hilldweller]

I'm still confused
Do I need a RCD?

I don't on mine.

"Fuse" is an entirely different matter, the inverter will shut itself down if overloaded.

 

[pyro]

There's some really excellent advice given here, so I won't confuse things.

Just to add that a well engineered true RMS (well, sinewave, anyway) will cost a whole lot more than a 12V fan, and a cheap low wattage double insulated hairdryer (look for 1970's 400W Moulinex ones on ebay) will work just as well on a £40 Maplins invertor as it would on a £500 quality job

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[Robin McHood]

@RodgerThat An RCD protects you from electrocution a fuse protects the equipment and cable from damage caused by overloads, totally different forms of protection.
But also what @Techno says above.
It is possible to have a different form of RCD which doesn't require an earthed neutral. Instead it looks at the current in both the "power" legs and trips if there is a difference of more than 30mA.
These are very common at sea as we tend not to have grounded neutrals at all, for reasons I wont bother confusing everyone with

 

[scotjimland]

It is possible to have a different form of RCD which doesn't require an earthed neutral. Instead it looks at the current in both the "power" legs and trips if there is a difference of more than 30mA.

can you explain what path is the leakage current is taking if there is no earthed neutral ?

 

[Robin McHood]

It means the current is escaping to earth via another route. You just dont want that route to be you!

Technically, on ships you don't earth neutrals to prevent spurious tripping, you might lose some vital equipment because of a relatively minor fault. So the neutral of distribution transformers, unlike ashore is never earthed down but left floating. Of course we still have to protect guys using portable mains powered equipment on deck, where electricity is particularly hazardous due to the presence of nice conductive seawater and sweat and loads of nice conductive steel around you

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[RogerThat]

Read the inverter instruction book

As per original post, I haven't actually bought one yet, still scoping out my options

Thanks for all your help everyone, interesting subject!

 

[tonyidle]

If neither line from the inverter is earthed then the only way to receive a shock from its output is by contact with both lines - a pretty unlikely scenario. Earthing one line so an RCD can be used is simply engineering circumstances where the risk of shock is increased so that the RCD is needed. Bit of a waste of time, money, and effort I would have thought?

 

[Robin McHood]

Nah sorry @tonyidle doesn't work like that.
The inverter output has a potential difference between each pole and ground, voltage is a measurement of that level. If you think of your ungrounded inverter output as two connected tanks of water above your head one for each pole.

They each have a potential energy. As long as its in the tanks you don't get wet, make a small hole in either one of them and the water will seek equilibrium (ground) and you will get wet.
The danger with any electrical circuit is a) voltage which provides the push and b) the amount of electricity that actually flows, this is the current. Its the current that kills not the voltage. The higher the voltage the more push and the higher amount of current that will flow. This is why personal protection RCDs are set to trip at 30mA as this is below the general heart stop current. The voltage is immaterial except in so far as insulation and sizing of internal components of the RCD is concerned.

Those guys who come in contact with HV lines arent so much electrocuted as burned to death by the vast amounts of current being pushed through their bodies by the high potential, heating the salt water part of their body so that they are boiled/incinerated from the inside out. I do have some lovely illustrative photos if anyone is interested ..No? Mmm thought not.

What you say sounds logical but you are not taking into account the capacitive charge that exists between any electrical source and ground whether one side of it is connected to earth or not, and you seriously don't want to use yourself to earth that charge!

On a larger scale it is what happens when you are struck by lightning, you act as the conduit between a high voltage source and earth potential even though the charged cloud is not earthed at any part of itself. The cloud exists at a potential difference of millions of volts to the ground. Harmless in and of itself but provide a conductive path to ground (ie a nice bag of thinly covered salt water = 1 human being) and its crispy fry time...

Anything above 30mA current is enough to stop your heart and the only protection a human naturally has is his/her skin resistance. This can be in the order of 100 thousand ohms or higher with dry skin but get wet, or wear a ring, or have a cut and that resistance drops dramatically, the amount of current your body will pass rises dramatically.

So for example in industry, generally you use stepdown transformers to take voltage from 220v down to 55-0-55v (centre-tapped to ground) which gives the 110v for actually operating the portable machinery but ensures that the operator will only be exposed to a 55v push, he'll still get a nasty shock and probably survive, but as I keep lecturing our guys at our safety standowns, the fall from the ladder as a result of the shock will still kill you....

The UK distribution system is one of the most dangerous around as it always has a 230v push behind it and it was designed that way to make electricity transmission cheap, especially to rural areas, as it does away with one expensive copper wire wire because you can send the neutral and the ground connections, literally via the ground you stand on, instead of a copper wire. On board our domestic suplies are at 220v the actual push if you come in contact with either live pin and ground is 110v cutting the push in half compared to the UK supply.

Here endeth RCD 101, Lesson 1 Chapter 1 ...may the force be with you....

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[Robin McHood]

I'm still confused

Do I need a RCD?

I don't intend running the extension lead outdoors and I don't use RCDs on extension leads that I have in my house.

But maybe that's the point? I have a fusebox on the house, I don't have with an inverter?

The fusebox won't protect you agaginst electrocution if anything you have attached to the inverter develops a fault. Its not there for that purpose. The fuse just stops things like cables overheating and catching fire when there is a fault, a 5A fuse for example will pass enough current to stop your heart about 150 times over, for the cost of it get an RCD & press the test button to make sure it works regularly!

 

[scotjimland]

The inverter output has a potential difference between each pole and ground, voltage is a measurement of that level.

as both @tonyidle and I both said already.. unless the neutral is connected to ground there can be no PD between each pole and ground.. and why there is no need for an RCD ..

the same use is made of isolating transformers for shaver sockets installed in bathrooms.. .. no earth.. no risk of shock from shaver. .. you could stand in a bath of water whilst shaving and be at no risk of electrical shock..

The output transformer is not earthed..

Broken Link Removed

 

[34127]

Adding a monostable multivibrator to the discussion - getting very technical now.

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[Robin McHood]

W

as both @tonyidle and I both said already.. unless the neutral is connected to ground there I can be no PD between each pole and ground.. and why there is no need for an RCD ..

the same use is made of isolating transformers for shaver sockets installed in bathrooms.. .. no earth.. no risk of shock from shaver to earth .

The output transformer is not earthed..

Just because a cat has 4 legs doesn't mean that everything with four legs is a cat. Probably just as well really, I'd hate to try and milk a cat...

You are making simple assumptions for what is actually a complex subject. I have tried to give a simple answer to the OP's query as to whether he should use RCD protection (of the correct type) for the output of his inverter.

He was not asking about whether he should use one to power his shaver from a circuit specifically designed to provide a very low current (around a maximum of 90mA) to a single double insulated device (class 2) in close proximity to the isolating transformer. Your shaver socket scenario has one major drawback in that a fault on the equipment can go undetected so I'd no more hold a mains powered shaver and stand in a pool of earthed water. While that 90mA limit may not kill me it will still be extremely unpleasant.

The situation he was talking about is very different and that's where the stray capacitances between phase and ground come into play. There absolutely WILL be a potential difference between the phase connections and ground. He was talking about extension leads which means potential of length = increased capacitance, plus multiple connected devices all of which contribute to increase the hazard with a power source designed to provide mains voltage at potentially lethal currents.

Protection against electrical shock hazards is not simple, it always combines and addresses a number of factors. The OP wasn't asking how, he was asking should I?

I attach for your info a paper by a member of my Institute which goes some way to explaining shock protection. I'd recommend reading ALL of it.

In a land based system and for example in @Techno 's van he has the neutral output grounded down correctly and uses an RCD to protect users of the output circuit. The reason for grounding the neutral is nothing to do directly with protection against shock hazard, it is to ensure that a piece of faulty equipment, where the phase contacts, say, the casing of a lamp, the associated circuit breaker or fuse then draws sufficient current to trip the breaker/blow the fuse and isolate the dangerous equipment.

The OP should wire his Inverter output like that IMHO and use an RCD. That way his van and himself are protected (as long as he isn't up a ladder when a fault occurs).

The inverter system you are describing is directly analagous to a standard marine supply system with isolated neutrals at the transformer . We do this because our imperative is to prevent what may be vital equipment on which the safety of the whole vessel depends from tripping off in the event of a fault.

Instead we have monitoring devices to tell us when such a fault occurs, and I spend an awful lot of time with my electricians tracking down such faults, because two such faults between phases can have very unpleasant consequences.

It does mean we have to ascertain the exact type of RCD we are using for the protection of people using portable power tools, as the tripping and test arrangements that work on a Phase/Neutral system will not necessarily work on a phase/phase system.

So I can assure you that there is a considerably increased risk the minute you start using extension leads and multiple devices, from a substantial power source, without RCD protection. Those capacitances will bite you....

Her endeth RCD 101 Chapter 1 Verse 2...

 

[Robin McHood]

Adding a monostable multivibrator to the discussion - getting very technical now.

Ahh now those take me back a bit, ever come across a vibrating reed oscillator or a multiple cathode neon voltage stabiliser.. you could tell if they were working correctly by the hum.. can't do that with these newfangled transistor thingies...

 

[34127]

Ahh now those take me back a bit, ever come across a vibrating reed oscillator or a multiple cathode neon voltage stabiliser.. you could tell if they were working correctly by the hum.. can't do that with these newfangled transistor thingies...

Do you remember the Avo valve tester - those where the days.

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[Techno]

I would read the first paragraph of page 3 carefully for some very well considered advice designed not only to protect but to cover their arse

https://cdn.shopify.com/s/files/1/0..._343a45e8-6e0d-49f8-b949-978ea43af453.pdf?731

 

[Techno]

To simplify
Sterling know that despite their best efforts they cannot force people to comply with using an RCD.
To minimise NOT remove the risk they supply it out of the box as centre tapped.
For those using an RCD you simple fit a fuse link to convert it.

 

[Louis]

So I'll be safe using a electric shaver in the shower aboard Brittany Ferries this weekend

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[Louis]

So I'll be safe using a electric shaver in the shower aboard Brittany Ferries this weekend

Only jokin boyo , I know serious topic and all

 

[Techno]

So I'll be safe using a electric shaver in the shower aboard Brittany Ferries this weekend

Not if it sinks

 

[Louis]

Oh sh-t never thought of that

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