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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Nitin Sidhu on February 15, 2015, 02:39:14 PM
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Hello!
A few questions propping up in my head.
We are moving into a space where diesel generator power, 3-phase, would be supplied to us instead of the 1-phase house power.
1.Mostly this would be a 4 core cable drop, without a grounding conductor available to the DG chassis or such. So Neutral bonding would need to be either local, or we carry a 100mts of single core to connect with the DG. What is correct ?
I would consider bonding the neutral with the ground at the main 3-ph distro we would then carry (for a start, a simple 4 pole isolator with a neutral and ground bus bond at the input, being fed to an 63 or 100 amp isolator)
2.What happens if the Neutral at the distro breaks ? Unbalanced circuit and a 440 to fry the equipment ? How to avoid ?
3. The max I see of CEE Form connectors out here are rated at 63amp, 5pole. Now, the amperage is ample for my needs. What I do not understand, is that I am sure a 125Kva genset is capable of delivering a lot more current that 63 amps, yet all I have seen here is a 63 amp receptacle on an input. What is the standard receptacle use for higher loads ?
I have some questions about RCCB/RCBO and local circuit MCB's that I hope to post soon too.
Thank you all for your time.
Regards,
Sidhu
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Ground and neutral need to be bonded in ONE place. This is usually at the first disconnect-so if there is a disconnect at the generator do it there otherwise at main distribution. Either way, there needs to be ground between the generator and the main distribution as well as ground rods at the generator and ideally at main distribution as well.
By DG, I assume you mean diesel generator?
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Obligiatory Disclaimer: I am not an electrician.
You should be able to have two bonding points, so long as there is only a single, common grounded neutral conductor between the bonding points.
At the generator, drive a grounding rod. Bond the grounding rod, generator frame, and neutral. (If the neutral is already bonded to the frame, bond only the ground rod and the frame.
At the distro, drive another grounding rod (grounding electrode conductor). Bond it to the EGC (equipment grounding conductor) and neutral.
There should be no other ground path (no EGC) between the distro and the genset besides the neutral, whether it be an intentional EGC/GEC or an incidental connection such as an audio shield, lighting kit supplied directly from the genset bonded to a truss that also has grounded audio equipment attached, or personnel that can come into contact with both grounding systems (distro and genset). It's a remote possibility, but incidental connections could result in ground loop hum, or personal injury or property damage.
(If you have more than one distro, such as one for audio and one for lighting, it gets ugly. Bonding at both distros could result in issues when something conductive comes between equipment connected to disparate distros. In that event, both distros must share a single, common bonding point, whether it be an EGC from each run back to the genny or an EGC run between the distros. This is a similar issue to what you would face when pulling power from two different homes for a street fair, as has been discussed previously in this forum. The homes have two different bonding points, possibly joined only by the neutral, which could be at different potentials.)
This is fundamentally no different than what is common for utility distribution, where neutral, ground (EGC), and ground rod are bonded at the utility transformer; neutral, ground (EGC), and a separate ground rod are bonded in the service entrance panel of the home, and there is only a single, common grounded neutral conductor between the transformer and the service entrance.
Of course, your AHJ has final say in the matter.
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Than you Stephen, I would *assume* that the neutral is already bonded to the ground at the generator panel, which is then also grounding the generator chassis. So if I could run a cable to the generator chassis, I would be good.
However, if that is not possible, then bonding our first disconnect is our best alternative.
Now, can I have the neutral permanently bonded in my main dsconnect ? This is ask if I do get a 5th grounding conductor also from the DG, then the Neutral bond is happening twice, at the DG and our first disconnect.
Thank you.
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Obligiatory Disclaimer: I am not an electrician.
You should be able to have two bonding points, so long as there is only a single, common grounded neutral conductor between the bonding points.
Ah thank you!
So a 4core cable feeding my distro, which would be grounded is good. Thank you again.
Regards,
Sidhu
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>:(
Than you Stephen, I would *assume* that the neutral is already bonded to the ground at the generator panel, which is then also grounding the generator chassis. So if I could run a cable to the generator chassis, I would be good.
This is a safety issue-do not make an assumption, find a way to test/visually verify. It is essential that the generators frame have path (bond) from the neutral.
While Jonathan is correct in that 2 bonds should work, here in the states utilities are governed by a different set of rules. The NEC, which most of the rest of us are governed by does not allow 2 bonds. Mainly because of the potential hazards caused by incidental or intentional metallic paths between the generator and the distro. So, if you do opt for 2 bonds, you need to take extra care in that area.
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Yes Stephen!
Thank you. My assumption is based upon the fact that i have not measured.
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I understand that in India, doing everything safely and properly is not always possible, because few people are knowledgeable AND concerned about safe and proper practices, so may be unwilling to put forth the effort and expense to do it right.
My hat is off to you, Nitin, for trying to do things right. The more you learn and the more you do, the more you will educate your peers. And to the extent that reduces injury, your world will be a better place.
(Cultural note: in Western culture, removing one's hat in the presence of another is a sign of respect. Historically, it signifies subservience or an inferior social status, such as a subject removing his head covering when in the presence of the king. As a gesture, it is similar to the military salute, which is initiated by the junior rank.)
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I understand that in India, doing everything safely and properly is not always possible, because few people are knowledgeable AND concerned about safe and proper practices, so may be unwilling to put forth the effort and expense to do it right.
My hat is off to you, Nitin, for trying to do things right. The more you learn and the more you do, the more you will educate your peers. And to the extent that reduces injury, your world will be a better place.
(Cultural note: in Western culture, removing one's hat in the presence of another is a sign of respect. Historically, it signifies subservience or an inferior social status, such as a subject removing his head covering when in the presence of the king. As a gesture, it is similar to the military salute, which is initiated by the junior rank.)
Haha! Thank you Jonathan! All good my man, even though we do have elephants walking around in cities, seriously, but remain otherwise quite well tuned into "western" culture.
That said, it is a pain here, and im constantly paranoid.
Cheers!
Sidhu
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And extension of the above situation.
So I built a small 3-ph distro, input 3p+N, grounded locally. Working well.
Now I am building a generator terminal box, this will have a 4pole terminal, feeding an isolator to a CEE form plug, further feeding the 3ph distro.
As I am only being supplied with a 4core cable (3p+N) from the power source, what happens if we get a fault here ? What protection to take here ?
Also, in 3phase systems, what happens if 2 live phase cables were to come into direct contact ? Will a 4pole circuit breaker trip ? Or does it rely on current flowing through the phase and Neutral wires only ?
Thank you!
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Breakers trip on current flowing through them-so a phase to phase fault will create an overcurrent that tips the breaker.
The reason for the G-N bond is to create a path for current to flow back to the neutral when there is a fault to anything that should not be energized. So, if staging or a piece of gear can conceivably come into contact with a phase conductor, either though a failed component or pinched wire, etc it should be connected to ground. The current then flows through whatever "equipment grounding conductors" it needs to until it gets back to the Ground-Neutral bond where it completes the circuit tripping the breaker.
So, if you draw out the circuit, you will see that the Ground and Neutral wires would need to be bonded at both ends of a "4-core" cable. As Jonathan pointed out this will function just fine-unless there are other metallic paths (and possibly even wet ground" between the two points. If that is the case, safety and function will be improved by added a 5th wire dedicated "safety ground" and bonding at one location.
For the casual reader, note that the "double" G-N bond is a special circumstance due to the OP being located in India and it requires extra diligence.. I doubt any AHJ in the States would allow it-and from a liability standpoint it would not be wise.
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Also, in 3phase systems, what happens if 2 live phase cables were to come into direct contact ? Will a 4pole circuit breaker trip ? Or does it rely on current flowing through the phase and Neutral wires only ?
Typically, breakers only monitor and protect the phase conductors. (Only AFCI and GFCI monitor the neutral; neutral and ground are never interrupted/switched.)
With a multipole breaker, an over current condition on any pole will interrupt all poles in the circuit. So a breaker on a 3-phase circuit will disconnect all three phases if one experiences a fault. If a neutral is overloaded, such as might occur with triplen harmonics introduced by switched-mode power supplies, there is no protection for the neutral.
To monitor and protect the neutral would require a multiple breaker that interrupts all phases in the event of a neutral overload, as interrupting only the neutral could result in a voltage imbalance on single phase, single pole (120V) devices.
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Typically, breakers only monitor and protect the phase conductors. (Only AFCI and GFCI monitor the neutral; neutral and ground are never interrupted/switched.)
With a multipole breaker, an over current condition on any pole will interrupt all poles in the circuit. So a breaker on a 3-phase circuit will disconnect all three phases if one experiences a fault. If a neutral is overloaded, such as might occur with triplen harmonics introduced by switched-mode power supplies, there is no protection for the neutral.
To monitor and protect the neutral would require a multiple breaker that interrupts all phases in the event of a neutral overload, as interrupting only the neutral could result in a voltage imbalance on single phase, single pole (120V) devices.
Thank you Jonathan! I more or less grasp it.
So what im aiming at is an insolator at the generator terminal, Circuit breakers at the main distro, Over Voltage protection devices after that (to protect against neutral faults), RCBO's at the sub distro's... This all gets very expensive very fast... haha...
A neutral breaking would only cause an overvoltage in a 3phase system, and not a single phase system. If i understand correctly. India is at 440/220v
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A neutral breaking would only cause an overvoltage in a 3phase system, and not a single phase system. If i understand correctly. India is at 440/220v
As I was rereading my post, yes, I can see that a broken neutral could cause an overvoltage condition in a 3-phase system.
Differences between North America and the rest of the world...
I don't know enough details about your systems in India. Here in the US, in addition to 3-phase, we also have split single-phase which is installed in every home, office, and factory to serve small portable tools, appliances, and lighting. (Homes never have three-phase; they all have split single-phase. Except for a few oddball install that *someone* is sure to mention. :-) )
In the split single-phase system, the secondary of the transformer is center-tapped, with the center tap going to the grounded neutral. That gives 120V between each "end" of the transformer and the center tap; and 240V between the "ends" of the transformer. In a 120/240V single-phase distro, the loss of a neutral could result in over/undervoltage conditions on the 120V circuits. Small portable tools, appliances, and lighting use 120V, while larger fixed tools and appliances use 240V (unless it's 3-phase in a commercial install).
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As I was rereading my post, yes, I can see that a broken neutral could cause an overvoltage condition in a 3-phase system.
Differences between North America and the rest of the world...
I don't know enough details about your systems in India. Here in the US, in addition to 3-phase, we also have split single-phase which is installed in every home, office, and factory to serve small portable tools, appliances, and lighting. (Homes never have three-phase; they all have split single-phase. Except for a few oddball install that *someone* is sure to mention. :-) )
In the split single-phase system, the secondary of the transformer is center-tapped, with the center tap going to the grounded neutral. That gives 120V between each "end" of the transformer and the center tap; and 240V between the "ends" of the transformer. In a 120/240V single-phase distro, the loss of a neutral could result in over/undervoltage conditions on the 120V circuits. Small portable tools, appliances, and lighting use 120V, while larger fixed tools and appliances use 240V (unless it's 3-phase in a commercial install).
Hey Jonathan,
During my first and only visit to the US a few years ago, I discovered that you had both 120v and 240v available at some venues. It kinda freaked me out. Now why would you have that ? Here, its 220/240, single or 3 phase.
I also managed to smoke a JCM900 (120v) head plugged into a 240v receptacle. I think this was at a D.C. club. Wasn't our head or crew, Bummer....
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During my first and only visit to the US a few years ago, I discovered that you had both 120v and 240v available at some venues. It kinda freaked me out. Now why would you have that ? Here, its 220/240, single or 3 phase.
Sorry, I'm not Jonathan... lol.
So many times for larger rigs, the amp racks have RackPacks of varying kinds; the Furman ACD-100 and the Peavey Distro are both good examples of introductory units that will serve their users well. I don't know for sure about the Peavey Distro, but the Furman can be wired either 1Ø or 3Ø, and accepts a 50A inbound connection. The distro then breaks that down into multiple 20A/120V circuits for amplifiers or whatever.
Larger rigs will use LEx Products or Motion Labs RackPacks; see attached for a JBL VerTec amp rack with a Motion Labs unit. That takes a 30A/3Ø L21-30 input and provides 4-20A breakers, one for each amplifier.
Large venues will usually just have CamLock disconnects.
Small to medium sized venues (the smart ones, who realise that their wall outlets might not be enough, and don't want production providers to have to run extension cords all over tarnation) will have both wall outlets and 1-2 30A to 50A/220V connections- the NEMA 14-50 "Range Plug" being the most common. Then, a provider can use that to power their amps in a much more efficient manner, and if a 20A breaker is tripped, it's local and therefore easy to reset. (Ever try to find a breaker panel at a school? And then find someone with a key to get in that room?!?)
Having both 120V standard 20A outlets for smaller rigs/stage power/backline, and 220V 30A-50A outlets for amps/monitors can make life easier for a production provider.
-Ray
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During my first and only visit to the US a few years ago, I discovered that you had both 120v and 240v available at some venues. It kinda freaked me out. Now why would you have that ? Here, its 220/240, single or 3 phase.
Legacy. You see, we invented this thing called "Electricity." Or, at least we like to think we did. In the beginning, Edison created light. (Please pardon my sacrilege!) He decreed that it should be 110V, and that it should be direct current. And he called it good. The disadvantage was that the generator had to produce 110V, and you couldn't push it very far before voltage drop made it worthless. That meant that every neighborhood needed a generator, and out in the country, every farm needed one.
Then along came Westinghouse (AKA "the evil one" if you were to listen to Edison), who touted the benefits of alternating current, chiefly because you could boost the voltage at the generator, send it long distances on wires, step it back down, and use it. Voltage drop became an insignificant factor. But since a light bulb didn't care if it was fed DC or AC, Westinghouse conveniently specified 110V AC. Edison's light bulbs would work just fine on Westinghouse's system. Generating plants could be large and centralized.
It didn't take very long for industry to progress beyond the light bulb. With motors and heating appliances, it was quickly discovered that 220V provided a more cost effective, yet still reasonably safe means of powering things. So we added 220V circuits, which could deliver twice the power across the same size wire. (And eventually, 3-phase and other voltages were developed.) But because we still had a huge installed base of 110V light bulbs and small appliances, we had to devise a system that would support the legacy 110V things. So we came up with the 110/220V split single phase system.
But since it isn't easy to dump that installed base ("I have 110V outlets, why do I have to buy a 220V toaster and rewire my house?") we've got so many "standards."
As someone once told me, "the nice thing about standards is that there are so many to choose from."
"But wait," you say, "why are you talking about 110/220V when it's supposed to be 120/240V?" That's because I was talking about the old days and over the years, the nominal voltage has gradually increased from 110V to 115V to 117V to 120V to 125V (and, correspondingly, 220V to 230V to 240V to 250V. No idea what happened to 234V.). [Not to mention the "130V long life" light bulbs you used to be able to buy.] Current NEMA standards specify 125 and 250V for wiring devices. Utilities generally aim to deliver 120/240V at the service entrance, though anything between 110-125/220-250V is considered to be within acceptable range. So anything within that voltage range is considered to be nominally the same voltage.
(Interestingly, the 208V found on three-phase wiring seems to always have been called 208V. But then I haven't done much historical research on three-phase voltages in North America.)
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(Deleted. I did not recall correctly. My apologies.) Mark C.
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Hello All!
Attached along is a simple diagram of our main 3-phase distro. A couple of questions.
>Does the neutral-ground bond have to be before the TPN or After the TPN ? Im confused considering that a TPN does not have protection on the Neutral pole, only isolation i think.
>Is it correct to use a TPN (three phase + neutral) MCB in this circuit, or should we be using a 4-pole MCB ?
To rephrase the thread, as we are only supplied a 4 core cable from the DG, we have to bond ground at our distro.
Thank you all for your time.
(https://3jiuew.bn1301.livefilestore.com/y2pb5sJ8yZ0oAlsd5zp78AfHoYQhv0I8BcHi64eJHxPn3GOKd_aXSK-oWzoYrdL2woqLYpVYx3JoQi-64eRR1HSpXTH7um3Gi4AZ04NVgCnz7vyyByoKgjAU4ozjnHFa6Su6-dBlO3JGKKk7KunceOddw/379px-TN-C-S-earthing.png?psid=1)
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Legacy. You see, we invented this thing called "Electricity." Or, at least we like to think we did.
Silly Americans... thinking they invented the world!!
Luigi Galvani and Alessandro Volta (Italian) put in most of the ground work, André-Marie Ampère (French) worked on electro-magnetism, Michael Faraday (English) invented the electric motor and worked on induction used to generate power.
Of course, Nikola Tesla (Serbian) showed the rest of the world what we should do with it.
As for Edison... http://theoatmeal.com/comics/tesla
Steve.
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Bonding prior to the TPN would be best-you don't want your bond disconnected when the TPN is opened.
As discussed earlier, it would be best if there were 5 wires between the genny and your distro. Lacking that, there should be a bond at your distro as well as at the genny; ie, the neutral should be bonded to the genny frame-otherwise a fault there can potentially raise the genny frame above ground potential creating a shock hazard.
Is there over current protection at the genny?
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Also, in 3phase systems, what happens if 2 live phase cables were to come into direct contact ?
Things would get very exiting! :o
There is 208 volts between phases so, yes, you would very likely trip the 3 pole breaker.
Probably some melted stuff to clean up as well .
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Edison didn't invent the light bulb. What he set out to do, and did was invent a system that would make home electric lighting less expensive and better then the gas lighting used at the time. To do this he improved the light bulb to the point that it gave off a reasonable amount of light and lasted a reasonable amount of time. He added to this the light base and socket, the fixtures, the wiring methods. switches, fuses the distribution system, the 3 wire shared neutral system among other parts of a home lighting system. He also hired and trained electricians to install all this and set up manufacturing plants to drive down the cost of light bulbs.
He got the AC DC thing very wrong no doubt about it he also didn't think to invent outlets for appliances. Overall he got a lot right and we all have benefited from it.
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Bonding prior to the TPN would be best-you don't want your bond disconnected when the TPN is opened.
As discussed earlier, it would be best if there were 5 wires between the genny and your distro. Lacking that, there should be a bond at your distro as well as at the genny; ie, the neutral should be bonded to the genny frame-otherwise a fault there can potentially raise the genny frame above ground potential creating a shock hazard.
Is there over current protection at the genny?
Thank you Stephen!
Yes, all gennies that I have come across have the frame bonded, and yes, they do have over current protection.
Im still trying to visualise the impact of the bonding at our distro prior to or after the TPN... Could you help me out here.. If the TPN is open, how would it make a difference ?
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If you had a phase to ground fault in the distro before the TPN (however unlikely, but it does/could happen) that fault should cause the TPN/OCPD to open. If the bond is after the TPN and it opens, then you no longer have a path for the fault current, but your distro box would be energized or "hot". With the bond before the TPN, the only way to clear the fault is for the OCPD at the genny to open-this removes power from your phase conductors preventing your distro from being a hazard.
If the bond is after the TPN, you might get lucky and the have breaker in the genny trip anyway-it all depends on the trip characteristics of the individual devices. It is obviously best not to leave safety up to chance.