Generator earthing/setup etc

[Bango87]

Hi all new to the forum and electrican. Working on a plant at the moment where the client is wanting to have two 480volt 60hz three phases motors installed inside there workshop. These motors are going to be fed from a generator as they do not have enough power to feed them off the workshop supply. So generator provided and a distribution board installed inside the workshop for isolation and protective devices for the two motors. The supply at the workshop is 415volt 50hz three phase and neutral. Now just have a few questions before I start.
1. I would like to run the motors at there nameplate rating so would set the generator up to supply 480volts 60hz. Will this be ok considering that the workshop supply is 415volts 50hz?
2. The generator will be sited outside the workshop and for earthing I am looking to do as follows. On the generator main switch generator side neutral connection has an Earth cable from there to the frame of the generator. I was planning on taking a Earth from this point half the size or more of the phase conductor to the distribution board sited inside the workshop then from the distribution continue the earth to the M.E.T of the workshop this will get the neutral grounded and also having a Earth at my distribution board. Then there are Earth bosses either side of the generator in the lifting frame which the lifting frame is bolted to the the generator frame. So I am going to take half the size or more of the phase conductor from one of the earth bosses to the M.E.T inside the workshop. Will this set up be satisfactory?
3. Setting of the MCCB of the generator. Generator has a 630amp MCCB. I am only needing to run 185mm cables from the generator. So settings of MCCB is as follows
- Io= In * 0.5 = 315amps rating is the breaker now
Ir= Io * 0.8 = 252amps trip point
Isd= Ir * 8 = 2016amps for starting inrush current
Motors are both 55kw. Is this set up correctly.
4. Generator has a RCD inside the generator. I am not taking a neutral out of the generator as motors do not require it my RCD at 30ma will still work as have link between Earth and neutral that is inside the generator connection from neutral gen side the the frame of the generator.
5. As limited Earth connections for on the distribution board that I have supplied with the cable I am installing planning on having the Earth cable from generator onto the earth boss inside the dist board. Then linking 0.5 or more of the phase conductor from that Earth boss to another Earth boss on the distribution board which is an Earth bolt then from there taking 0.5 or more of the phase conductor from outside the enclosure onto this same bolt to the M.E.T of the workshop. Is this satisfactory?

Is this set up all satisfactory? Sorry for the long message be much appreciated if can answer my questions.

Kind regard

 

[waross]

No problem setting the generator for 480 Volts, 60 Hz. Don't forget to change the UFRO jumper on the AVR to the 60 Hz position. Your no-load frequency should be 61.8 Hz, or 1854 RPM.
When a large motor starts the speed and frequency may drop before the governor reacts to correct the speed.
A drop in frequency upsets the V:Hz ratio and can lead to magnetic saturation of motors and transformers. To avoid saturation and to aid in frequency recovery the Under Frequency Roll Off feature of the Automatic Voltage Regulator acts to reduce the voltage almost in proportion to the drop in frequency.
If the either the UFRO frequency jumper or the no-load speed are improperly set, the voltage stability or the ability to recover from block loading may suffer.
Grounding and protection are subject to local codes. Where in the world are you??


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Bango87]

Hi bill thanks for getting back I'm in the Britain and the job is in Britain?

Kind regards

 

[Bango87]

The earthing arrangement inside the workshop is a TN-S system. Earth coming off SWA cable. What about 3-5 aswell? Cheers

Kind regards

 

[waross]

I'm in Canada. Hopefully one of our UK members will respond.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Few thoughts -

3 - make sure your generator can actually deliver sufficient current to trip the breaker under short circuit conditions. Many can't.
4 - I think a 30mA RCD could be troublesome in this application. If you can clear an earth fault without using it AND you aren't relying on it for personnel protection AND you're not going to end up with a TT system then consider disabling it.
5 - difficult to say without seeing the installation, but generally a short direct path to MET is the preferred solution. Is the earth provided by the utility or is it a rod / mat?

 

[Bango87]

Thanks Scotty for replying. The workshop DNO supply is a TNS earthing system. The workshop consumer unit is directly behind the generator other side of the wall inside the workshop. And I am going to be positioning my distribution board next to the workshop existing distribution board about 3meters along the wall where there is space. So my plan is to take a 185mm 4core (three phase and Earth) to my distribution board too supply the motors using the black core as my Earth. This earth within the SWA will connect to the Earth connection on the generator which is a bolt in the frame of the generator and is also linked to the neutral aswell. The earth core at my distribution board will be connected in the earth boss inside distribution board. Then from another Earth boss on my distribution board take a 185mm Earth cable from this boss to the workshop M.E.T. Then I am going to take a Earth bond from M.E.T to the lifting frame of the generator 0.5 of the earthing conductor. Is this acceptable? Do you think the settings of my breaker is wrong? Have tried to set it lowest possible which it is set as low as it can on both Io and Ir. With the Isd I set it at 8 which can go lower but would it be necessary with this setting?

Kind regards

 

[waross]

There are two types of earthing.
System earthing and equipment bonding.
The system should be earthed with a connection to the neutral wye point, either directly or through a current limiting impedance.
There shall be one and only one connection between the neutral and the earth.
Equipment grounding is to avoid dangerous potentials with respect to ground from appearing on the surfaces of equipment. Equipment grounding provides equi-potential zones to protect workers. Different from system grounding, there may be more than one equipment bond or ground connection. This is what the lug on the generator frame is for.




Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Bango87]

Yeah so the set up that I have will be correct then. 185mm from generator Earth connection which is linked to neutral to my distribution board and then to M.E.T then bonding down the lifting frame to Earth aswell to keep all metals parts at the same potential? Is this correct?

Kind regards