Local Bonding conductor - Sizing 1

[NickParker]

What is the IEC /NEC code to determine the size of the ground cable (PE) for the equipment frame grounding to the nearest grounding bar (local bonding)?
This is in addition to the ground cable that runs along with phase conductors from the source to the equipment.

In my case the size of the ground cable which runs from the source is same as that of phase conductor. Should the local bonding cable be same size or be downsized?

 

[Gr8blu]

NickParker: Classic answer - it depends. If the source is an adjustable speed drive (doesn't matter whether it's AC or DC, as long as it has some type of rectifier in it), for example, and the equipment frame forms part of the ground circuit between rotating member and ground plane (think ground brush connected to end bracket connected to motor stator frame or foot, and then by conductor to facility ground plane), then the ground conductor needs to be both large in diameter AND short in length. This is due to the high-frequency switching going on in the power source, requiring a very low impedance path to ground - and the presence of "skin effect" on the conductor. Usually this becomes a bonding conductor at least the same cross-section as the current-carrying phase conductor - and sometimes up to 2x the section, depending on the switching frequency of the electronics.

For general use applications, I believe the correct NEC reference is Table 250.66 (which basically applies a bonding conductor that is somewhat smaller than the largest phase conductor).

Converting energy to motion for more than half a century

 

[dpc]

In NEC-land it will depend on the type of equipment. For typical electrical equipment, the equipment grounding conductor (EGC) that runs with the phase conductors is all that is required. If this is a distribution panel or switchboard taking service from the grid or a panel in a separate building, then an electrode grounding conductor is required. Sizing is covered in Article 250 of the NRC. But if it is sized the same as the phase conductors, it should be OK. About IEC requirements, I don't know.

 

[che12345]

dear Mr. NickParker (Electrical)(OP)15 Nov 22 13:55
"....#1. What is the IEC /NEC code to determine the size of the ground cable (PE) for the equipment frame grounding to the nearest grounding bar (local bonding)? "[/color]
1. (a) IEC term PE is an "earthing " conductor.
(b) NEC term "grounding " conductor , a conductor connecting to [earth].... But what is "earth " ; the [soil] of this planet or where? Consider on board of vessel/air craft/ car etc. , they do not have any connection with this planet [soil].
1.1 In IEC, PE conductor Spe =Sph up to 16mm[sup]2[/sup]; Spe=16mm[sup]2[/sup] for Sph 25-35 mm[sup]2[/sup]....etc.
1.2 In NEC see NEC section 250 for detail.
" .... #2. This is in addition to the ground cable that runs along with phase conductors from the source to the equipment."
2. This may be considered as a :
2. (a) IEC term "equipotential bonding conductor " may be equal to earthing PE conductor size but may not need to > 25mm[sup]2[/sup] of copper material.
(b) NEC term " equipment bonding jumper on load side ..." See Table 250-122 for detail. Attention not < No.14.

[COLOR=]"...#3. In my case the size of the ground cable which runs from the source is same as that of phase conductor. Should the local bonding cable be same size or be downsized? "
3. The local "bonding " conductor sizing , see above 2.
Che Kuan Yau (Singapore)

 

[7anoter4]

Since a grounding conductor has to be "loaded" only in a grounding fault case the rating of a grounding conductor is connected with short-circuit phase-to-ground or phase-to-ground-to-phase current and protection clearing time.
If the system neutral is solidly grounded the current is high but the time is short. Let say the supply conductors withstand the short-current then if the EGC is the same it is o.k.
Since the short-circuit current which will be a part of an equipment it is not easy to calculate, there are practical rules to choose the right EGC.
So, NEC-2000 presents these rules in art.250 Part VI. Equipment Grounding and Equipment Grounding Conductors from art.250.109 and Part V. Bonding from art. 250.90 -for low voltage equipment. For above 1000 V from 250.190 Grounding of Equipment.
In U.K. according to BS7671
BS 7671 -2008 CHAPTER 54 EARTHING ARRANGEMENTS AND PROTECTIVE CONDUCTORS
Regulation 543 PROTECTIVE CONDUCTORS [NOT FOR BONDING GROUNDING]
Chapter 544 PROTECTIVE BONDING CONDUCTORS
Regulation 542.3.1 for protective conductor buried in ground
BS presents a way to calculate the grounding conductor cross section area.

 

[FacEngrPE]

What is the IEC /NEC code to determine the size of the ground cable (PE) for the equipment frame grounding to the nearest grounding bar (local bonding)?
This is in addition to the ground cable that runs along with phase conductors from the source to the equipment.

The green wire run with the phase conductors must be sized per the NEC (in the NEC world). As indicated above it's primary purpose is to ensure a low impedance for fault returns so that proper operation of fault protection is ensured.
Sizing of bonding cables depends on purpose, and is not (as far as I am aware) covered in the NFPA 70 National Electric Code (NEC).

Some possibilities (Not a complete list)
[ul]
[li]Touch and Step voltage control, IEEE Std 80, Guide for Safety in AC Substation Grounding[/li]
[li]NEC Hazardous Area 1 or 2, to reduce energy available for ignition during a fault.[/li]
[li]Static Electricity Dissipation, NFPA 77 Recommended Practice on Static Electricity[/li]
[li]Welding Return Path ANSI Z49.1, "Safety in Welding, Cutting and Allied Processes,"[/li]
[/ul]

My point being - you need to decide what the bonding conductor needs to accomplish, before it's size can be determined.