Electrical Distribution board design and connection philosophy 2

[Michael2009]

Hi Sir,

I have plant provide source 480V,4wire,3phase,60Hz,TP&N Neutral solid to earth. I need to provide a distribution board distribute power to 3 phase motor A and B and one 3 phase heater
I have few question on distribution board design.

1. TP&N neutral solid to earth incoming is mean my distribution board will only have L1 L2 L3 and PEN for incoming and distribution connection to motor and heater will be only 3 wire (L1,L2,L3) and earth terminal motor connect to local ground directly? We do not need to bring motor earth terminal back to incoming PEN right?

2. And the distribution board PEN terminal can be wire to enclosure ground and wire direct to local PE also?

3. what is the best way to step down the voltage 120V AC single phase. My initial design is wire L-N (3phase) to get 277V AC single phase then step down to 120V AC using step transformer. Any other better way?



Appreciate and thank you very much for you guys spending time to reply my question...

 

[1capybara]

Michael, please correct me if im wrong, but your background is in BS7671 or IEC and you want to design this to NEC,
US electric code in a country that requires it like KSA, correct?
Ok to answer your questions:
1. TP&N neutral solid to earth incoming is mean my distribution board will only have L1 L2 L3 and PEN for incoming
and distribution connection to motor and heater will be only 3 wire (L1,L2,L3) and earth terminal motor connect to local ground directly?
We do not need to bring motor earth terminal back to incoming PEN right?
ANSWER: a. Distribution panels must be grounded as close as possible to the panel if they are the main panel for the building. Are you doing that?
Are there any neutral loads originating at the panel?
b. If ground is under 25 ohms and the GEC (grounding electrode conductor, the wire to the earth spike or ground rod) is sized per NEC article 250, no objections.
2. And the distribution board PEN terminal can be wire to enclosure ground and wire direct to local PE also?
The PEN terminal bar should sit directly on the panel enclosure, with no insulation between them, so no need to separately ground the enclosure, just run the GEC directly from the terminal bar to local ground.
3. what is the best way to step down the voltage 120V AC single phase. My initial design is wire L-N (3phase)
to get 277V AC single phase then step down to 120V AC using step transformer. Any other better way?
no, no other way less expensive.
Finally, is this job going to inspected by an inspector and have you asked him the above? Because his word is final.

 

[Michael2009]

Hi 1Capybara,

I was refer to IEC standard. Will it have any different?

The application are for industry plant

My DB incoming have five terminal , L1, L2, L3, N and PE. My client provde 4-wire 3phase. I will connection wire L1 to L1 terminal, L2 to L2, L3 to L3 and N to N. Then my Incoming PE terminal will tap to enclosure/ body DB. And Body DB will bring to local ground near to DB.
Is this correct way to do it?

Now the question coming on distribution to my motor rating 37Kw and 22 Kw.

Should I give 4 wire to my 3 phase motor? Using L1, L2, L3, and PE. Which mean bring my PE from motor enclosure terminal ground back to DB PE is this correct? Or just 3 wire for 3 phase and PE terminal inside motor enclosure shall ground on local motor side.

No inspector on this. I am trying to design as best industry practice.

 

[unclebob]

What are PE, DB and PEN ?

What I understand:

You want to connect two 3 phase motors on a 3 phase 4 wire switchboard:
Use the 3 phases on the motors. No neutral. Ground motor's body.

 

[ScottyUK]

Bob,

PE - protective earth
DB - distribution board
PEN - protective earthed neutral

PEN is a combined neutral and earth conductor connected to earth (ground) at the source. It is frequently used by UK utilities for LV distribution, but its use is prohibited for non-utility networks.

Michael,

Your location must have national codes and regulations, and these generally take precedence over IEC documents. You need to find your local codes, or at least state where you are in case anyone on here knows those codes. You might find the Cahiers Techniques papers #172 and #173 helpful from an IEC / European perspective. They are listed under the Low Voltage tab.

 

[jraef]

I find it odd that you are saying this must conform to IEC standards, yet the voltage is 480V and the controls are 120V. That is typically only a North American standard.

Be that as it may, a motor does not need the Neutral (PEN) connection in 99.9999% of applications (I've only heard tell if one that supposedly did). It is however considered prudent to run the Ground (PE) connection from the motor to the DB ground bar. If there is a VFD on the motor, I also recommend connecting the motor ground to the building ground in order to have as short a path to ground as possible for any Common Mode Noise, but that is not the norm for motors connected Across-The-Line (DOL). As to how any of that relates to IEC regulations I'm not sure.

And by the way, the typical way to get 120V control power here it to connect a Control Power Transformer (CPT) directly to the 480V, not 277V. You will find it more difficult to find a 277-120V transformer, but 480-120V transformers are everywhere (here).


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[Michael2009]

Thank you all for your help.

 

[paulengr]

Just because it's not required does NOT mean its not a good idea.

Based on the description the overcurrent protection is also the ground fault protection since the neutral of the transformer is solidly grounded. This is an extremely common U.S. configuration as mentioned. However it often has a huge problem in that a ground fault can persist indefinitely causing much greater damage than is necessary.

This is called a peg ground. It's illegal in many jurisdictions for good reason. The scheme is where everything is wired up with a 3 wire system and then a local ground stake (or just tying to building ground) is used.

The problem is that in a line-to-ground fault, the most common type (over 90% of faults). It is extremely common in motor circuits in particular because a mechanical fault in almost all cases wipes out the windings in the very thin air gap (1 mm in a 200 HP motor for reference). So we need to make sure that we have adequate ground fault protection.

If the ground path is say 25 ohms (more or less a standard) then the maximum ground fault current will be 277 / 25 ohms = 11.1 A. This current is so small that it will only trip the overcurrent protective device on very small motors (usually fractional HP). It will NEVER trip in larger motors at least until the damage reaches a point where it destroys both phases and enters either a L-N-L or L-L fault at which point the current is sufficient to trip the overcurrent protective device.

The situation improves if with a separate ground (4 wire) directly to each junction box, motor casing, etc., so that we can maintain say 2.7 ohms or less in which case the fault current increases to 277/2.7 = 100 A. But for larger motors this is still less than the normal full load current of the motor and thus not recognized by the overcurrent protective device as a fault, but for many applications for motors under around 25 HP it will suffice.

The only true solution to the problem is to use ground fault protection in the motor starter for larger motors. And this automatically provides all the infrastructure needed to go ahead and run a high resistance ground which in turn eliminates damage from ground faults altogether as well as eliminating arc flash hazards from ground faults and does not cause excessive fault voltages unlike ungrounded deltas. The cost for a 480 V system is typically under $1000 US as long as you avoid a couple companies that heavily advertise this stuff with grossly inflated pricing.