One point earth and fuse size with length 1

[Elie2]

Hi,

I have 2 separate questions:
1. A project specifcations say that:

a. The transformer neutral shall be earthed to a bar, one tip of which linked to an earth pit and the other tip linked to the earthing ring around the premisses.

b. The neutral in the MDB shall be earthed (they say to help with the short circuit current quick trip of the breakers), it shall be earthed to a bar one tip of which is linked to an earth pit and the other to the earthing ring aound the premisses.

I do not want to do this for 2 reasons:
a. This creates an earth loop.
b. This causes all LN faults to become LE faults, but LL faults remain of the type LL. The breaker is a PACB Masterpact from Merlin Gerin equiped with a Micrologic 5.0 unit which does not sense faults to earth.

So am i right by not following these specifications?

Question 2:
I read somewhere that something needs to be done when feeder length become excessively long and become very resistive and reduce the short-circuit current which can delay the fuse or breaker disruption. I forgot what need to be done except that the solution with the software i use is to increase the size of the breaker. Please can you point me to NEC or other litterature about this in case i want to do something by hand or as per a table?

For those who use ETAP, we have a licensed station but the program is too complicated, any easy introductions?

Thanks.

 

[rbulsara]

1. The project spec is correct. Your reasons are not valid. Its OK to creat a earth loop between Tx and the service entrance. Do not bring earth conductor from Tx to the switchboard, just the neutral. (Think, there is no breaker between the Tx secondary and the service disconnect.)

By the way the neutral and earth should also be bonded to the Tx enclosure.

When I have more time, I may elaborate or there are many generous people here wo I am sure will post their views.

Sounds like you need to familiarize your self with Codes and grounding. Please read up more on grounding including IEEE Green Book and NEC 250.


2. Breaker upsizing is even more counter productive, larger breaker needs more current to operate. Plus you can not apply a breaker/fuse exceeding ampacity of the conductors. If the fault current is low you can set your earth fault or instantaneous settings more sensitive. Only way to verify this with some confidence is to use a software!

With all due respect, not using ETAP or any software because you find it complicated is not an acceptable excuse. For your own good, learn it if you want to excel in this field.

 

[Elie2]

I am sorry, for item 2: the solution with the software is to increase the size cable, not the breaker as i said above. Actually what it tells me is that it was unable to find a suitable breaker, i increase cable size manually and lock it and run simulation again and it gives me a breaker. The distance is 300m and the load is 80Amps, the new sizes which give acceptable voltage drop have a much higher current carrying capacity (cable is 95mm2, breaker is 320A which is a lot more than the load, it is as you said for the cable).

What i want to know is there some trick when it comes to long distances, or just treat it like it was 20m long. I think that the earthing conductor needs to be increased with the cable: fases increase to reduce voltage drop, breaker increase to protect the cable and earth increase to increase the fault current and reduce tripping time, but how to calculate this? I would just add resitances in series and calculate the value but i want to see it written somehere if possible.

Thank you for item 1. I will sure work the way as it is. My worry was from another installation, where the breaker had a Micrologic 6.0 which used to trip because some current ran divided in N and E and thus the RCD used to be fooled and tripped. I think something should have been done, either the RCD be moved lower in the diagram (i just set it trip to highest), that link could have been removed ...

 

[mc5w]

Also, circuit breaker cost is about in 1.5 power proportion to the amps. That is, a 1600 amp circuit breaker costs twice as much as a 1200 amp circuit breaker all other things being equal. Thus is the reasoning behind allowing up to 6 disconnects for a service or transformer secondary.

 

[rbulsara]

Elie2:

For item 2.: You mention the problem at other site. That happens because the neutral current sensor is at the wrong place. Draw a picture of the CTs and N and E connections. Make sure that the bonding jumper between the neutral in the service disconncet is "before" the N sensor (that is between the sensor and the Tx). All of the load N current shall go thru the N CT and it can split after that.