Ground Grid Study

[tctctraining]

If the customer didn't have specific instructions on whether the cacluation be done for a 70 KG or 50 KG person.
Which one is recommended to choose?
Is there a difference of paractice for NEC or CSA point of view?

 

[WindTurbinesAreFine]

It would be more conservative to choose 50kg body weight since they can tolerate less current.

Your best bet is to follow IEEE-80-2000 if this is for a substation application. I assume you are in Canada?

 

[tctctraining]

Yes , I am in Canada.
Cost difference between 70 and 50 is another criteria that needs to be considered.
if 70 KG is considered, less copper and Rods will be used.

 

[WindTurbinesAreFine]

Yes, it is true that you can get away with less copper if you use 70 kg as your design criteria. However, depending on your conditions, you could have unsafe conditions for persons weighing less than 70kg.

You would be best served to take the conservative route for the sake of safety than to take the less conservative route for cost.

Have you run any studies yet? Did you have to add a significant amount of buried copper to meet 50kg criteria?

What software are you using? What is the application of this grid?

 

[tctctraining]

Ground grid is for substation. I am thinking if possible to use 70 KG for inside the sub and 50 KG for around the fence.
We use CYMGRD.

 

[WindTurbinesAreFine]

If your software will allow it, that could be a practical solution. You might want to recommend that signage be placed around the substation not permitting persons less than 70kg to enter the sub yard as there is a risk of a potentially lethal step/touch voltage.

 

[tctctraining]

signage is a good idea but what about the substation fence?
If the calculation is done for 70 KG, the touch potential will be higher for a 50 KG person when touching the fence.

 

[jghrist]

Do you have a company policy that prevents people weighing less then 70 kg from entering the substation?

If not, I see no justification for using 70 kg inside the substation and 50 kg outside.

 

[tctctraining]

jghrist,
makes sense.Thank you.

 

[azadani]

Have you choosed prpoper shock time ? this can affect the amount of copper cosiderably. How about unsymmetrical grid design, have you tried it?Isolating fence may help your copper saving depending sub. area.

 

[WindTurbinesAreFine]

Isolating the fence is not a good idea if you have overhead lines that cross over the fence. A line to ground fence on an ungrounded fence would surely kill a person.

I agree with the statement of shock time affecting your results. You have to choose one that is realistic, yet conservative enough. We typically use 250 milliseconds.

 

[tctctraining]

The shock time is set by the software at .5 second. I always use this!!!
I beleive this depends on the total clearing time of upstream breaker.

WindTurbines..
Can I ask, based on what assupmtion you use 250ms?

 

[jghrist]

The shock time is set by the software at .5 second. I always use this!!!
I beleive this depends on the total clearing time of upstream breaker.

WindTurbines..
Can I ask, based on what assupmtion you use 250ms?

The shock time should equal the fault duration which depends on relay and breaker interrupting time. There are many nuances with breaker failure, reclosing, etc. I suggest you read the relevant section in IEEE Std 80 that discussed these nuances. 0.5 seconds is a lot of time for a high fault current. On what basis do you use 0.5 seconds? Surely the software will allow changing this.

 

[WindTurbinesAreFine]

The software we use actually is preset to 250 milliseconds. We also compared this with the clearing time of our upstream protection and found that 250 ms is actually slightly on the conservative side anyway. We recently did a study using 500 ms when the client specifically asked us to. The results were dramatically different.

 

[tctctraining]

CYMGRD default is 0.5 seconds, If there is a reclosure action then the results would be different and clearing time needs to be higher.

 

[WindTurbinesAreFine]

True, which is why the clearing time shouldn't be chosen at random. This is one of those little inputs that has a huge effect on the outcome.