Why system neutral should be grounded at the power source only? 2

[tem1234]

Hi,

I don't see what will happen if a system neutral is grounded at 2 or more points and what's the danger. Can you explain me that? In many reference it is written that you should not do that, but why? If you have any good reference about this it'll be very appreciated.

Thanks

 

[davidbeach]

Define system. What voltage level are you talking about? There are uni-ground systems and there are multi-ground systems.

In a uni-ground system the ground is intended to be a non-current carrying conductor and the neutral is intended to carry all normal imbalance current. Multiple connections between ground and neutral puts neutral currents onto the ground. Ground fault detection that assumes all ground fault current on the ground and none on the neutral will not function properly if there are multiple connections.

 

[tem1234]

I was talking in general, but to be more specific, i would say a 347/600V 4 wires at the secondary of a Dyg transformer. You should ground the system neutral at the transformer only (or nearest as you can).I see that some neutral current will pass throught the ground if you ground at other place, but can this create dangerous voltages at some place?

 

[gepman]

You also have no control of where the current goes once it is in the ground system. The currents can cause problems with some types equipment

 

[DanEE]

Maybe you have to see it to understand it..

When you put a current probe over conduit and see load current flowing on it, you better appreciate what tying neutral to ground in more than one place can do..

Removed the second tie of neutral to ground and problem resolved.

 

[cranky108]

There is another issue here in that during a fault the different grounds can create a step potential hazard.
I will admit that at 600v and less it is not very likely, but that is some of the concern in the NEC rules.

Haveing said that, in large spread systems it becomes difficult to ensure, and maintain single point grounding.
I've seen 480v systems extending a quarter mile in each direction from the source.

 

[ScottyUK]

In the UK our utilities use a technique called PME - protective multiple earthing - which earths the neutral conductor of an LV distribution circuit at multiple locations throughout the system. Loss of any one neutral-earth connection should not adversely affect the safety of the system. This link gives some explanation:

http://books.google.com/books?id=uc...6_jr&sig=En7c1oux81Kl4_ygL7hfzvM5yMs#PPT61,M1

PME does not allow the use of a fast acting earth fault protection scheme using residual connected CTs or a core balance CT. On the other hand, any phase-earth fault implicitly becomes a phase-neutral fault with a resultant healthy fault level which results in rapid clearance. Use of PME in the UK is limited to the distribution companies: larger industrial consumers with a dedicated transformer will have a TN-S supply which does allow use a fast acting earth fault protection.


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If we learn from our mistakes I'm getting a great education!

 

[cranky108]

Still single or multi grounded systems don't reduce the risks in high impedance grounds. And at 600v or less this is usually limited to the extents of the business customer.

At the 120v level there has been an improvment with ground fault indicating(GFI) breakers and outlets. But such systems require an isolated neutral for that circuit. This may also apply to 240v as I don't keep up with all the new products.
The arc fault indicating(AFI) breakers may also save some lives, but I haven't studied these to know the requirments of the neutrals.

There are new relays that are made to detect arcing faults, but these are used mainly in power systems. Besides I have heard reports that these are less than 50% effective.