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#180212 - 08/17/08 09:12 PM Network meters; network service  
junkcollector  Offline
Member
Joined: Apr 2007
Posts: 36
Central MN
I've heard about buildings in heavily loaded downtown areas served with what's called a "network service." To me, I look at this setup as as a system of large vault type transformers installed in the street that are paralleled onto the secondary mains and connected with a special switch called a "network protector." I've seen a special kind of electric meter called a "self contained network meter" and needs a fifth terminal? What is the difference between a 120/208 Y network service and a standard 120/208 Y service? I don't understand why a service like this needs a special meter. Perhaps I've got it confused.

If anyone can explain this to me I'd be most greatful. Thanks.


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#180215 - 08/18/08 10:05 AM Re: Network meters; network service [Re: junkcollector]  
winnie  Offline
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Joined: Sep 2003
Posts: 649
boston, ma
You may be encountering different uses of the term 'network service', with entirely different meanings.

I have seen the term used to describe 120/208 'single phase' service, such as might be used for individual residences in an apartment building. Such a service is really two legs of a 208/120 wye, and so at the basic physics level is not really 'single phase'; but is used in the same fashion as a residential single phase service.

As to why two legs of a 120/208 service requires a different meter from a 120/240 service: A standard meter for 120/240 single phase service 'cheats' and is not 'blondel compliant'. The 'blondel theorem' basically says that to correctly meter a three wire service you need to make _two_ voltage measurements (say the two leg-neutral voltages), and _two_ current measurements (say the two 'hot' leg currents). A standard 120/240 meter simply makes one voltage measurement (the leg-leg voltage) and then assumes that the leg-neutral voltage is exactly 1/2 of this value. In other words the meter ignores any phase difference or imbalance.

Because of the phase difference present in a two leg 120/208 service, such a meter would read anywhere up to 14% low, not accurate enough for utility metering.

-Jon


#180218 - 08/18/08 06:26 PM Re: Network meters; network service [Re: winnie]  
junkcollector  Offline
Member
Joined: Apr 2007
Posts: 36
Central MN
Thanks for the explanation.cool

Basically what you are saying is that you need a meter with two voltage coils, and have each coil connected between the neutral and each hot leg. I would imagine that the neutral would get connected to the sideways fifth terminal, instead of with a standard 120/240 v where the meter is not connected to the nuetral at all (one voltage coil that gets 240, connected to L1 & L2)

That makes sense. Thanks again.



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