CT cabinet with PT cabinet

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James L

James L

Senior Member
Location
Kansas Cty, Mo, USA
Occupation
Electrician
I'm freelancing for a week or so, getting ready to set a 480/277 service, and am perplexed by the gear described vs. the riser diagram

On the print, the description says 400a meter, but the diagram shows CT cabinet, PT cabinet, and designated "M" for meter.

A couple of questions...

1) what is a PT cabinet? What size would it be?

2) how would PTs work in conjunction with CTs?

🤔🤔
I tried to attach an image, but file size too large 😕
I may try to attach later from my computer
 
zbang said:
PT - Potential Transformer
Some metering schemes need them, but I've never see that on a 400a service (CT's, yes). Could be for an energy monitoring system.
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I was wondering if it's power power quality monitoring.
It's just a retail store 🤔

So, a PT cabinet is what?
just an empty cabinet?
Maybe with a back plate like a CT cabinet?

Or a cabinet with PTs in it already?

Riser diagram looks like I would nipple out of CT cabinet?

Any idea why print describes 400a (320a cont) meter?
 
Usually with energy management, they just request a pulse meter. Usually with the PT’s, (just a step down transformer) is for powering the meter, which may be 120 even though the service is 480.
 
James L said:
I was wondering if it's power power quality monitoring.
It's just a retail store 🤔

So, a PT cabinet is what?
just an empty cabinet?
Maybe with a back plate like a CT cabinet?

Or a cabinet with PTs in it already?

Riser diagram looks like I would nipple out of CT cabinet?

Any idea why print describes 400a (320a cont) meter?
Click to expand...
From what I am seeing from searching, it looks like it is to step down the voltage at the meter as a safety measure for the utility person if the need to access the meter. Low amp and low voltage instead of high. Some of what I read stated to get an accurate measurement of the voltage, but may be mostly for safety at the meter.
 
Bluegrass Boy said:
From what I am seeing from searching, it looks like it is to step down the voltage at the meter as a safety measure for the utility person if the need to access the meter. Low amp and low voltage instead of high. Some of what I read stated to get an accurate measurement of the voltage, but may be mostly for safety at the meter.
Click to expand...
I wondered about the meter voltage.
 
hillbilly1 said:
Usually with energy management, they just request a pulse meter. Usually with the PT’s, (just a step down transformer) is for powering the meter, which may be 120 even though the service is 480.
Click to expand...
Thanks. That makes sense, for sure. I had one line of thought trting to veer that way but couldn't find anything online to support the idea. Most everything I could find looked more like power quality monitoring.

I'll try to call POCO to see if they want it, and what kind of cabinet they're looking for
 
James L said:
Thanks. That makes sense, for sure. I had one line of thought trting to veer that way but couldn't find anything online to support the idea. Most everything I could find looked more like power quality monitoring.

I'll try to call POCO to see if they want it, and what kind of cabinet they're looking for
Click to expand...
If we have PTs on a service we generally mount them to the side of the CT cabinet(inside). For that one I could see 2.5:1 PTs. The old meters couldn’t handle voltages above 120.
Today’s meters can handle 120-480 on the same meter (kV2c for instance). Still, some POCOs do not want voltages in the meter base higher than 120V for safety concerns. Some meter techs aren’t used to working with voltages over 120
 
Standard metering in North America is 120 V nominal for voltages and 5 A for currents. In Europe the current is 1 A. So we use PTs and CTs to make this happen. The ranges and accuracies are different for metering and protection so sometimes you see both. It will usually have its own fusing on PTs and shorting blocks on CTs for maintenance purposes. Most of this stuff comes prebuilt as a packaged assembly as part of a switchgear order.

What they are hooked to, the metering side, can be as simple as a couple selector switches and a meter or as sophisticated as a power quality analyzer, revenue meter, and protection relay. Maybe for customer use or in some cases the POCO has customer side metering off the low side of the transformer.
 
Our local utility requires a PT cabinet for most 480V services. Its just another box on the side of the CT cabinet.

The utility specs the size of the box, and other parameters, and they install transformers in it.

Whoever manufacturers your CT cabinet will usually have a PT cabinet option available for a specific utility.
 
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