120/240 ungrounded res 1950's home

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aaatraker

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hooking up ac unit at 240v on ungrounded system, how do i do this safely?
my thoughts were that a ground wire [ equipment ground} in the romex that i use is attached to the frame of the unit just as in a grounded system, and run back to the neutral terminal, that way fault current has a path back to the source the utility transformer. you don't need earth to trip the breaker just a path to the source correct?

code refs and procedures please

thanks
kurt
 
Re: 120/240 ungrounded res 1950's home

I think we have to take time to be clear about our use of terms, for it is very easy to misunderstand one another if we use the same words in different contexts.

What do you mean by saying the system is ?ungrounded?? Are you saying that there is no ?ground bus? in the main panel, and that there are no equipment grounding conductors anywhere in the facility? Or are you saying that there are no ground rods or other grounding electrodes, and that the voltage on the phase conductors floats at an unknown voltage above earth potential? I have seen the term ?ungrounded? used in both ways.

If the answers to the two questions above are ?yes? and ?no,? in that order, then I think your solution would be acceptable from a physics point of view. However, the ?grandfather clause? concept might come into play, and cause you problems. Since you are touching a key element in the system, your local inspector may require that you bring other aspects of the system into compliance with current code requirements. They may, for example, ask for GFCI or AFCI protection, in appropriate rooms, and justify the requirement on the basis that you are creating, for the first time, a ground connection to which such devices could be connected. I would call that a stretch in logic, but I am not an Inspector.

My best advice is to discuss this question with your local Inspector, before you make any commitments to the owner.
 
Re: 120/240 ungrounded res 1950's home

Good advice from Charlie. We are confused on grounding and bonding because the NEC is not always technically correct. I suspect that what you mean is the wiring system does not have an equipment grounding conductor, which is not the same as ungrounded. However, there are ungrounded 240 V systems around.
 
Re: 120/240 ungrounded res 1950's home

I have not seen the system, a friend at work is doing this at his house. He does not have grounded outlets and says there is not a ground rod or water pipe ground on the system. I have avoided giving direct advice because I don;t want to impact my license by telling him one way and he doing it wrong and having problems. But the way housing is selling out here in ca i might run into this problem myself on a job, lets assume its the way he says. Would you need to change the system to a grounded system to do it correctly?

thanks
kurt
 
Re: 120/240 ungrounded res 1950's home

If indeed there is neither a ground rod nor any other connection from the neutral (officially entitled the ?grounded conductor?) to planet Earth, then I would say that you must add one. I would not say that you must then run equipment grounding conductors and convert every receptacle to a grounded (i.e., 3-prong) outlet. But the Inspector may require you to run an EGC to any new circuit you add, and to any existing circuit that you touch.
 
Re: 120/240 ungrounded res 1950's home

aaatraker
to add recpts' to a system with no ground do one of the following:

1. Make the home run recept box a GFCI recpt and install outlets on the load side with all of these outles being labled as "no equipment ground". The cords plug in but THERE IS NO GROUND.

2. Run a seperate EGC (green wire) to any portion of the grounding electrode system (read to ground rod or bonded H20 pipe)and install the new grounded outlet.

3. add only 2 prong outlets.

4. pull new wiring
 
Re: 120/240 ungrounded res 1950's home

what is the difference in this system and one that is in a ship or plane neither has a earth ground but circuit breakers trip by there being a connection to neutral at the generator or source with the equipment ground wire right? so the breakers should trip on fault with a connection to the transformer through the neutral groud connect correct as in earth can not be the only ground?

thanks
kurt
 
Re: 120/240 ungrounded res 1950's home

Earth does not play much of a roll if any at all in the function of the breakers or fuses. mater of fact if you were to connect a hot(ungrounded) conductor to a grounding electrode (rod) that had the required 25 ohm's resistance to earth the circuit would just sit there with 4.8 amps or 472 watts on it (which by the way wouldn't trip a 5 amp breaker). Grounding electrodes are only there for two functions: Lightning strikes and to limit voltages from higher voltage system's. Like when a tree limb falls and causes a 7.2 kv line to land on your service drop. This would trip a breaker or fuse because it would deliver a 288 amp or 777,600 watts of power @25 ohms. So if you want a little protection from lightning or higher voltage then install a grounding electrode system but remember the NEC is bare minimum and there are better ways to make a more effective system. but to allow for the operation of breakers and fuses in the event of a short in a appliance then use the method that you described by running the EGC back to a neutral bonded connection. and as others have said check with the local AHJ that might allow or disallow this.
 
Re: 120/240 ungrounded res 1950's home

The difference between a ship and a land-locked facility has nothing to do with the circuit breaker?s ability to trip upon sensing a fault between a hot leg and the neutral. The difference has to do with the possibility that a fault might occur between the hot leg and the case of a tool or item of equipment. The current path will be from the source via the hot leg to the tool, from there via the short to the case of the tool. The key questions are (1) whether there will be a complete path back to the source, and (2) whether that path has a low enough resistance to permit a high enough current to terminate the event by tripping the breaker.

Wayne has accurately described the situation for a land-locked facility (such as the house you are working on). But let me describe a slightly worse situation. A person is holding a tool that experiences a fault from hot to the case. The current path will go from the case of the tool to the operator?s hand, through their body to their feet, to the floor, into dirt, along dirt to the ground rod, and up the ground rod back to the source. The resistance of this path is higher than just the 25 ohms of the ground rod?s connection to planet Earth. The path also includes the resistance of the person's body. The current will be lower than the 4.8 amps, so the breaker will not trip. However, the current will be higher than the 0.10 amps necessary to kill the operator. That is why the Equipment Grounding Conductor is installed. It connects the case of the tool directly to the grounded point of the source. In the event of the fault described above, there will be a total of three current paths: (1) The normal path through the tool, (2) The path through the operator?s body, as described above, and (3) A low resistance path through the EGC. Path #3 will allow enough current to quickly trip the breaker and terminate the event.

Now let's talk about a ship (I can?t claim knowledge of aircraft). They are looking for an additional level of reliability. They don?t want a single ground fault to cause a trip of a vital circuit, because it may cause them to lose steering or some other important capability. So they leave the system unconnected to the hull (i.e., ungrounded). If one ground occurs, what you get is a grounded delta with that phase becoming the ground point. Nothing trips yet. It would take a second ground fault to cause a breaker to trip. In the mean time, they have ways of detecting, locating, and repairing the first ground fault, and hope to be able to do so before the second ground fault occurs.
 
Re: 120/240 ungrounded res 1950's home

aaatraker
A very safe electrical system is a 2 wire system (with no equipment grounding [bonding] conducotor) and GFCI protected. This system is actually safer than a "grounded" system, as with no ground (green wire) there is less potential for a ground path. You can still get a fatal shock with a GFCI if you are in series with the hot and neutral. In fact the NEC allows replacing a 2 wire non grounded receptacle with a 3 wire GFCI with no equipment grounding conductor.
 
Re: 120/240 ungrounded res 1950's home

And Charlies reply about a ship is interesting. Even though its an "undgrounded" electrical system, you can still get a fatal shock due to the capacitive coupling. By ungrounded we mean one of the conductors is not intentially bonded to earth. this is not the same as the ungrounded wiring you are referring to.
 
Re: 120/240 ungrounded res 1950's home

Thanks to all for the information, There are lots of these 1950s and older homes in my area without grounding, two prong outlets only. So how to make a safe install on equipment at 240v, does come up.

kurt
 
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