OT: 12 VDC to 240VAC converters

Gentlemen,
I have acquired a 12VDC to 240VAC converter which I have mounted on a flat board. I have run it and tested it and am pleased with the output which is 500
watts continuous but if pushed will put out 1000 watts for a short period. It is an industrial type and has low battery, over temperature and overload alarms. The 240VAC comes out of the unit into a 6 Amp trip which cannot be set unless there is voltage and an RCCB. My question is that it has an earth lead that comes from the earth common rail inside the trip box, where the converter earth and the plug socket earth all meet, that goes at the moment no where. is this the earth for the earthing rod I have ?
Martin P
--
snipped-for-privacy@btopenworld.com

Semper in Excrementem Altitudo Solus Varius
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sat, 22 Jul 2006 17:20:11 +0100, campingstoveman wrote:

Is one of the "phases" from the invertor also bonded to that earth point? Is the invertor earth connected to the neutral and earth pins of the output socket?
What are you intending to power with this device just a single appliance directly connected via it's lead or more than one via distribution block of some sort?
--
Cheers snipped-for-privacy@howhill.com
Dave. pam is missing e-mail
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Without taking the inverter to pieces I cant answer your question, a friend dropped by since I posted this and he says as it came out of a vehicle it would have been bonded to the vehicle I do also have an earthing rod, brass and four foot long.
Martin P

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hi.. If it came from a vehicle this would have been bonded to the chasis to avoid any part of the body becoming live, The RCCB is not the same as an RCD a rcd requires an earth where as a RCCB measures the balance of the phases, i.e as soom as whats going out does not match what is coming back (when you accedently slice a cable with a spade and get a shock) it will trip, I would allways earth the device as for these devices to work there needs to be an earth loop .. check this quote from a website....
"The 240 volts from an Inverter can kill you instantly, just like the 240 volt at home, so you need to take precautions. All modern houses have a Safety Switch (or Earth Leakage Circuit Breaker - ELCB or Residual Current Device - RCD) to cut off power quickly if someone touches the 240 volt while earthed - but many Inverters do NOT have them. Safety Switches will only protect against electrocution to earth if you connect the Earth Terminal on the Inverter to an earthing rod that has a good earth connection - a rod into dry sand is useless. "
HTH
Bob
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Being very picky only the old type of ELCB requires an earth connection to it to make it work, neither a RCD or a RCBO does . Unless buying second-hand and very old stock it is not possible to buy a device in this country that needs the ground connected to it to make it work. They are easy to spot as they have a separate earth terminal or wire However if both sides of the supply are floating i.e. one side is not bonded to the earth then RCDs, RCBOs etc. will not function correctly, they may do but there is no guarantee.
--

Julian Tether
Barge Parglena
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Yes it is !
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
campingstoveman wrote:

Martin, There are two ways of running this device. One way is to connect the earth terminal to a good earth and duplicate the system in your house. If you contact the live lead and earth it, the RCCB will trip. The neutral is bonded to earth within the unit. This is OK in a fixed instalation. If you're using the inverter in a portable situation, you can run it isolated from earth. Do not make any connection to earth or any exposed conductive part. In order to get a shock, you now have to touch both the neutral and live conductors. touching either one alone is not a problem as there's no earth return for the current to take. As there's no earth, the RCCB is redundant but the chances of touching two conductors is slight so the risk is reduced. This is how the majority of portable generators are run. I think fairground rides are run this way as well. Hope this helps.
John
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
This was my understanding too. Put plainly, if using a generator, you are better off not using an earth post as this just makes it easier to complete the circuit if a wire is damaged. It's harder to get across two wires than live to earth.
Is that correct?
Regards,
Kim Siddorn
john manders wrote

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Kim Siddorn wrote:

That's quite correct. Both ways are recognised in the IEE regs. ISTR that running earth isolated is not recomended for fixed installations. Here an earth fault on either conductor can go un-noticed but it effectively connects the system to any earthed metal after the RCCD. The remaining conductor is then live (even if it's labled 'Neutral') and is not protected from giving a shock. By only using this on a temporary portable installation, the likelyhood of such a fault occuring is reduced. There is also the occasional problem of providing an earth point. Hammering in an earth rod is not always an option.
John
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 24 Jul 2006 04:30:50 -0700, John wrote:

Ish...
Or worse the chassis of all appliances connected could also be live relative to their local ground depending on where the fault is. Overload protection devices don't activate as there the no fault current via a low impedance earth path.
For the vast majority of appliances out there it's probably safer to bond one of the generator phases to the generator chassis and a good local earth spike and the "neutral" pin of the output socket. This emulates the normal single phase domestic supply of Live, Neutral and Earth for which the vast majority of appliances out there are designed to operate with. The wise also fit a current operated RCD at the generator.
--
Cheers snipped-for-privacy@howhill.com
Dave. pam is missing e-mail
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
campingstoveman wrote:

Is this going to be a fixed installation or portable?, it changes the answer considerably.
Greg
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Portable in my van.
Martin P

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Mon, 24 Jul 2006 22:10:35 +0100, campingstoveman wrote:

In that case I'd bond that "earth" point to the vehical body work very securely in an electrical sense.
A proper RCD, that is a current operated one, doesn't care two hoots about earths precense, abscence or "proper" return paths. All it worries about is that X amps went to the load and X amps came back if the difference is more than its rating it trips. Remember that an impedance of 8,000 ohms will allow 30mA to flow at 240v, so the path doesn't *have* to be a good one and "it's volts that jolts, mils that kills".
Voltage operated trips should be tripped with a big hammer and replaced with current operated ones.
--
Cheers snipped-for-privacy@howhill.com
Dave. pam is missing e-mail
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Dave Liquorice wrote:

I can't agree with that, if you're going for the floating supply without an earth rod then the last thing you want to do is bring large lumps of metal into the equation. If you stick to one appliance at a time then even if it has a fault there's only one piece of metal to touch so no path, introduce more pieces of metal and the possibility of touching two things thus forming a circuit arises so you have to get into proper equipotential bonding.
Of course if a genie is plugged into the mains inlet of a camper you have no choice as the earth pin on the socket is required to be bonded to the van body so as to trip a protective device if the body becomes connected to live because of a fault in the fixed wiring in the van. But if just operating an inverter inside a van there is no such reason to do it.
In fact if you buy one of the small inverters on the market today you probably won't be given the choice, they have a floating output socket for a single appliance and no terminal for connecting anything else.
This issue is similar to the over enthusiastic equipotential bonding that was required by the 15th edition of the regs. Electricians went mad bonding every bit of metal they could see in almost every situation that involved water, it must have cost the country a fortune and can still be seen in toilets, kitchens and laundry rooms across the country. Then it was 'realised' that this was making things more dangerous not less, and the 16th edition now only requires in it very specific and limited situations, namely rooms with a bath, shower or pool where people are going to be immersed. Everywhere else it's recommended to leave pieces of floating metal isolated so they can't form part of a circuit.
Greg
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Greg,
I already have and use a small inverter in my van to run my laptop and the instructions say use earthing point and it has one.
Martin P

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
campingstoveman wrote:

I have a 250W one in my camper and it doesn't have one so there must be variations between brands, it just reinforces the point that it's a very grey area.
Greg
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Right Martin, a fixed installation would have been easier to answer 8-). The regs for fixed genies are fairly straight forward, in short the accepted interpretation is simply that you earth the neutral so it mimics a normal mains supply and you can use all the normal methods of protection.
Unfortunately the regs for fixed static inverters and portable genies together only cover half a page, and there are no regs at all for portable static inverters.
For portable genies they say that you must fit an RCD in a TN, TT or IT system, and as all three systems require an earth, and indeed an RCD won't work without an earth, you can infer the need for an earth rod. This is, however, contentious as an earthing rod rammed six inches into dry sand as you see all too often with portable genies is completely useless, they have to be made properly and tested which clearly isn't going to happen in many cases.
Makers of small genies like Honda do not bond the neutral to their ground terminal, it's just the frame of the set so connecting to an earth rod is fairly pointless as the output is still floating. They clearly believe that a floating output is better than an earthed one with a useless earth rod, and if you only use it to connect direct to something like a TV or single power tool I completely agree with them.
The problem arises with bigger installations, like catering vans, campers etc, where a loose live wire could touch the case of something like a cooker. This on it's own would not present a hazard if the supply was floating, but neither would it trip any protective device so it would go undetected until the next electrical test. Now if, in this time, a neutral wire came loose in say a water heater and touched it's case, there would be a very serious risk to anyone touching both metal cases simultaneously. To reduce the risk in this situation you now have to use proper equipotential bonding between all metal parts that can be touched, both the appliances and the van itself.
This situation is also violating a basic principle of the regs that a fault should not go undetected but should trip a protective device, in fact it's a specific requirement for genies in general that they should use EEBADS, which in short means an earth so that it will trip, but it doesn't say that under the section on portable genies or static inverters!.
Then there's the problem of installing and testing an earth rod, it's no good stabbing a bit or rod six inches into dry ground and hoping!. It typically has to go in a meter and testing requires specialist kit. Most electricians have kit that can test it if there is a mains supply available, i.e. and earth fault loop impedance tester, but most don't have the kit to test it without a mains supply as this is rather specialist.
To cut a long story short, a properly installed and tested earth rod along with an RCD, bonding of the neutral to earth and equipotential bonding between all metal parts of the van is the safest way. But in the real world the earth rod isn't going to be worth a dam so leaving the output floating is the best bet, just try and limit it to one appliance at a time. I had an inverter in my camper and did exactly this, I kept it completely separate from the mains hook-up which is intended to get an earth from the mains.
Greg
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Tue, 25 Jul 2006 00:29:05 +0100, Greg wrote:

Key words "earth fault loop impedance", generally is refering to getting the earth loop impedance down low so that suffcient *fault* current can flow in a live to earth fault condition to trip *overload* devices in the specified time. This means it has to be very low particulary if the overload devices are wired fuses rather than MCBs. A 30A wired fuse takes a considerable overload, several times its 30A rating, to blow it quickly. This means the earth loop impednace has to be very low, typically only an ohm or three to allow these massive fault currents to flow.
The requirements for shock protection are not the same. The "leakage path" can be any route and be fairly high impedance, an 8,000 ohm "return" path is enough for 30mA to flow and a typical RCD to trip. Bringing down the impedance of the earth return path can only be a good thing, better to have 5k provided by a dry rod into shallow ground than 10k of intermittent chassis contact or much more if the generator has rubber wheels...

Agreed, though with an RCD fitted the requirement for a low impedance earth rod is not essential. Indeed the regs insist on a "whole house" 100mA time delayed RCD in situations where there is no supplied earth and the earth fault loop impedance via a locally provided spike cannot be made low enough for fault protection.
Generators, invertors and earths is a minefield of stuff with lots of variables affecting other variables that knock on to others and back to where you started... My view is to bond "generator" chassis to "neutral" and bond that to a reasonable real earth connection at the generator and feed all loads through a 30mA RCD (to provide shock protection) and suitably rated overload protection device (to protect the generator).
--
Cheers snipped-for-privacy@howhill.com
Dave. pam is missing e-mail
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Dave Liquorice wrote:

I think you misunderstand what I was saying, you can use an earth fault loop impedance tester to test an earth rod if a mains supply is present. This is a standard practice since any electrician has to have one already so it avoids buying more kit, it's taught on the inspection and testing course. This doesn't mean they expect the earth rod impedance to be anything like as low as that of a mains supply where an earth is provided by the board, in fact it will typically be a hundred times more.
For example I have a Fluke multifunction tester but chose not to buy the earth rod testing option as it added 150 for something that I didn't need, and I know of 2 other electricians with the same kit who made the same choice. The only time you need a proper earth rod tester is when there isn't a mains supply present, in which case you have to use the 3 electrode method which involves driving another two rods into the ground some distance away from the one to be tested and connecting all three to the meter.

Yes they do insist on an RCD for TT installations, and they also insist on a measured rod impedance of less than 200 Ohms in order to allow for changing ground conditions and corrosion between inspections. A few K may theoretically be enough to trip a 30mA RCD but if you accepted such a high reading then what happens when the ground conditions change or the rod corrodes?.

Indeed, even more so when you parallel with the mains, by the way I design such systems for a living and as we speak am developing a G59 module for our product range.

For a fixed installation I totally agree, but for a portable one the problems of getting a good earth on a camp site make it highly debatable, how do you know it's low enough to even trip an RCD?, quite simply you don't so can't rely on it. The fact that the most popular maker of camping generators, Honda, don't earth the neutral speaks volumes.
Greg
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Thanks Greg and all,
I have mounted the inverter on a piece of suitable timber and only hang it in the van for convenience, I also have a camper and a caravan which have mains and gennie use which in the former use the site earth etc. Its been an interesting thread.
Martin P

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.