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Old January 26th, 2007, 01:58 AM   #1
rajy2r
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Question Current Transducer

I was trying to attach a pic but for some reason it wont let me. Anyways i zipped it and attached it.

My question is regarding the grouding of terminal 3. I was curious as to why that is done. From what i can tell its for reference, but correct me if i am wrong. Could someone give a better explanation if there is to why that is done.
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Old January 26th, 2007, 02:41 AM   #2
Ken Moore
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I posted your pic for you.

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Old January 26th, 2007, 04:45 AM   #3
Sparkz
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This relates to grounding transformers...

EN60204 states that the secondary windings of any transformer (so including current transformers, like the CT in the drawing) have to be grounded for safety reasons.

Edit: If it's the first time you've come across CT's, please be extremely careful as to shortcircuit the secondary before disconnecting the CT from the circuit. Or make sure there's absolutely no line current flowing through it when disconnecting.

Last edited by Sparkz; January 26th, 2007 at 04:53 AM.
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Old January 26th, 2007, 05:44 AM   #4
Andybr
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Expanding on Sparkz closing comment you should still shortcircuit the CT secondary before disconnecting it even if you are absolutely sure there is no current flowing. Just because there is no current when you disconnect it does not follow that the circuit will not be re-energised. The open circuit secondary voltage on a CT can be extremely high and is quite capable of causing injury or damage.
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Old January 26th, 2007, 07:59 AM   #5
Tom Jenkins
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The output voltage of a CT with an open lead is the ration of the CT to the line voltage. So, if you have a 500:5 CT on a 480 volt line the potential is 4,800 volts.

From personal experience, that represents a significant shock hazard!
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Old January 26th, 2007, 10:39 AM   #6
Sparkz
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Sorry Tom, but I was told that a current transformer will raise its secondary voltage to ANY level, just to get the correct current flow through its secondary, that is, according to its ratio. That's why CT's are supposed to be shorted when not connected to an ampmeter or the device (kWh meter?) mentioned above.

It would also explain why CT's with a 20:5 ratio burn when the secondary is left open. Leaving the secondary open is not only hazardous to personel, but it will definitely destroy the CT. I've seen CT's like this burn out in a matter of seconds on a 230V generator, just by leaving the clamps open, so the line voltage doesn't really matter.
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Old January 26th, 2007, 11:06 AM   #7
TConnolly
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Eureka! Ya' never know what you're gonna to find here...

I've been purchasing power transducers from one supplier that we are very happy with, but they don't offer quite what I need in a multi-variable transducer for one particular application. So I googled the part numbers off the picture to find out who made the OP's instrument and gave them a call. Crompton has something that might just be the ticket.
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Old January 26th, 2007, 11:20 AM   #8
Andybr
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Alaric.

I have used many Crompton transducers in the past and they are perfectly OK but have you considered using an Energy Meter with some kind of serial/network connection. I currently use Merlin Gerin PM710MG meters (also available as Square D) on Modbus and they provide all of the information you would normally require (including TDH). I have used other similar instruments including some made by PML which are equally good but the MG units are comparable in price to a KW transducer alone.
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Old January 26th, 2007, 01:37 PM   #9
Gerry M
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Quote:
Sorry Tom, but I was told that a current transformer will raise its secondary voltage to ANY level
I believe Tom is correct, except it should be 48,000 Volts(480*500/5). That definitely is a hell of a significant shock hazard. I know in theory the voltage can go to infinity, but that isn't the reality based on many factors, such as internal coil resistance and core saturation, etc. Either way its not good not to short.

Last edited by Gerry M; January 26th, 2007 at 01:53 PM.
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Old January 26th, 2007, 03:14 PM   #10
Tom Jenkins
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Quote:
Originally Posted by Gerry M
I believe Tom is correct, except it should be 48,000 Volts(480*500/5). That definitely is a hell of a significant shock hazard. I know in theory the voltage can go to infinity, but that isn't the reality based on many factors, such as internal coil resistance and core saturation, etc. Either way its not good not to short.
You are absolutely correct, Gerry - my math was off by an order of magnitude!
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Old January 26th, 2007, 03:22 PM   #11
rajy2r
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Thx. for the help guys. Appreciate it.
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Old January 27th, 2007, 08:50 AM   #12
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Hi Tom & Gerry (?!),

The line voltage is totally inherent to a current transformer! The AC line voltage can have any value from 0V to 1000V (some types even more), as long as the current passing through the CT does not exceed its ratio.

Sorry, but I can't see the relation between the line voltage and the open clamp voltage of the CT.
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Old January 29th, 2007, 02:46 AM   #13
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Oops, in stead of 'inherent' I meant 'independent'.
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Old January 29th, 2007, 07:48 AM   #14
Tom Jenkins
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The voltage of the lline defines the potential energy the CT has available to push current across the opent circuit. This relationship can be found in some of hte CT manufactuer's literature.
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Old August 14th, 2007, 07:58 PM   #15
arroyo
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I'm looking for the correct size CT (current transformer)
I want to install (1) CT to monitor the amps of one leg of a 3 phase 460 volt 125 HP motor with a full load amps of 167. I have an analog ampmeter scaled from 0 - 150 amp, and a CT with a 300 to 5 ratio. , please advice if this will work, or do I need to make some changes.
thank you for your help

Last edited by arroyo; August 14th, 2007 at 08:00 PM.
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