Protection vs. Metering 1

[tulum]

Hello,

Is it appropriate/advantageous to place an ammeter on the same ct as an instantaneous overcurrent relay and a motor protection relay ( all burdens in series)? i.e. metering and control on same ct?


Thanks in advance...

 

[scottf]

It really depends on the purpose of the ammeter and the burden it places on the protection circuit.

From an ammeter perspective, if the purpose is general indication, then it should function ok using a CT rated with a protection accuracy class. If the ammeter is for revenue metering, then you would be well advised to connect it to a CT with a revenue metering accuracy class.

Inserting the ammeter into the protection circuit will place an additional burden on the CT. If the ammeter is electronic, then that additional burden will most likely be inconsequencial. Either way, you'll need to confirm that the additional burden of the ammeter will not cause the CT's accuracy class burden to be exceeded.

On a reliability note, what are the consequences of the ammeter failing? If this relay will trip off a line when the ammeter fails shorted acress the CT (i.e. no/little current makes it to the relay), then it may not make sense to put the meter in the same circuit as the relay.

 

[ScottyUK]

Something else to consider is how your ammeter will behave if the monitored circuit experiences a heavy fault. A protection CT will comfortably deliver 10 or 20x rated current if a suitably large fault current exists. Will the ammeter survive this, or fail open circuit? If it fails open circuit, you have a dangerous condition which will cause high voltages to be developed which may damage your CT.

-----------------------------------

Start each new day with a smile.

Get it over with.

 

[scottf]

Good point Scotty...you're correct about the dangers of failing open circuited. I need to recall my last paragraph above...I was addressing CTs and thinking VTs at the same time!

 

[jbartos]

Suggestions: The ammeter incorporation into the protection circuit should be reconsidered for the following reasons:
1. If the ammeter fails, it will compromise the protection circuit, which might have serious consequences.
2. The ammeter incorporated in the circuit safely would require its safe incorporation additional hardware, e.g. an auxiliary transformer.
3. Generally, the metering is often located in the metering cubicle and the protection in the protection cubicle.
4. Etc.

However, when it comes to motor applications, the ammeter may be viewed as a luxury item, and any cost cutting measures may prevail.

 

[Alwaysexciter]

It is very common to have an ammeter in the same circuit as the protection device. It is usually not economically practical to have separate CT's for simple indicating meters.

I have never had a situation where an ammeter failed during a fault. If this is a concern then an ammeter selector switch could be installed and kept in the OFF position except for viewing loads.

Note: if you’re installing a modern solid state motor protector relay, a separate ammeter will not be required since these units give a current display.

 

[CJFlatters]

If you want some friendly advise based on personal practical experience - keep your protection and instrumentation on separate systems.

1) Protection CT's have a greater saturation level than instrumentation CT's, it is therefore probable that a hefty fault would stuff your meter.

2) I have operated an ammeter selection switch on an old Crompton Parkinson ACB (this being connected into the protection CT un beknown to me) - result, CB tripped and production line shut down with significant down-time and re-tooling cost.

Plus you already have some very valid technical responses from scotty et al above, on the subject of the response from "Alwaysexciter", I have been made aware of an instance where an intermittent ammeter fault kept causing spurious tripping during starting of a large pump motor (3.3kV 150kW DOL) in an industrial process.

_______________________________________
Colin J Flatters BSc(Hons) IEng MIEE MIIE
Electrical Engineer / Project Manager
Email - cflatters@colin7.demon.co.uk.

 

[stevenal]

I'll agree with alwaysex here. It's commonly done, just check your burdens. Probably a standard someplace, but the ammeters I use have withstood numerous faults using protection class CTs.

 

[ScottyUK]

Interesting disparity - the two British engineers are shying away from doing this, and the cowb..., sorry, American engineers are saying it is ok.

I wonder if there are significant differences in the robustness (is that a word?) and overload withstand capacity of meters from our respective countries? There certainly are noticeable differences in other equipment, where the US-origin equipment tends to be big 'n' beautiful compared to the European equivalent which is usually just big enough to meet the spec. On the other hand, I think the average European equipment is superior in terms of shrouding of live terminals, and sometimes in design technology where sophistication is used rather than brute force.

I'm not advocating one or the other, before all the US engineers shout 'Foul!'. There are numerous items of equipment we have installed where the brute force solution of a US-manufactured part has been far better than the European equivalent, and where we have gone to the trouble to source the US part.




-----------------------------------

Start each new day with a smile.

Get it over with.

 

[eglaude]

I have to agree with Alwaysex as well. With electronic devices incorporating metering and protection togehter in a small, relatively inexpensive package, it's probably worth spec'ing the gear with these types of devices up front.

If cost is that much of an issue, the standard arrangement of O/L's - shorting block - AM switch is the ticket.

 

[stevenal]

If you are uncomfortable with current loops disappearing inside a box, Satec has a solution. Pass your 7.5 mm CT cables through the CT cores mounted on the back of the box. Never used them, but they might satisfy the open CT concerns.

http://www.oksatec.com

ANSI metering standards do not preclude CTs from meeting protection class requirements as well. Good way for CT manufacturers to reduce inventory, just send the right set of curves along. I regularly specify dual class CTs. What else would you use with a protection device that also claims ANSI metering acurracy? Are IEC metering CTs required to saturate during 10X faults?

 

[jbartos]

Comment: If the ammeter happens to be a voltmeter measuring voltage across the shunt, and having TVSS, then there will not be any problem. However, I would recommend the modern motor feeder current monitors with various useful features, if there were funds for luxuries.

 

[jbartos]

Suggestion: Visit

http://www.thomasregister.com

and type Monitors: Voltage & Current under Product or Service, which will return 82 companies to approach to for motor current monitors

 

[CJFlatters]

I do appreciate the cost of installing extra CT's for general instrumetation is often not practical / necessary.

My recollection of my first instance is that the culprit was an ammeter selection switch, not an actual ammeter which must have momentarily open-circuited the protection CT to trip the breaker.



_______________________________________
Regards -

Colin J Flatters
Consulting Engineer & Project Manager

 

[jbartos]

Comment on the previous posting marked ///\\My recollection of my first instance is that the culprit was an ammeter selection switch, not an actual ammeter which must have momentarily open-circuited the protection CT to trip the breaker.
///This is one way to engineer and design such an ammeter in the circuit. Another way is to use a millivoltmeter, shunt resistor and TVSS to protect the millivoltmeter.\\\

 

[jghrist]

stevenal,

Yes, IEC metering CTs are required to saturate. They have an instrument security factor, Fs, that indicaties the multiple of rated current at which the metering core will saturate. It is given as a maximum value and is usually 5 or 10.

 

[scottf]

jghrist-

Not to split hairs, but not all IEC metering accuracy CTs have a security factor, especially ones used for indication metering, i.e. cl 0.5 or cl 1.0. Unless something is rated like Cl. 0.2Fs5, the CT is not assumed to saturate quickly.

Also, it should be noted that the security factor is only valid if loaded at the rated burden. The saturation point increases linearly as the burden decreases, i.e. half the rated burden means twice the rated security factor. Most CT specification I run across tend to specify a much higher burden than is seen in most modern installations.

 

[stevenal]

Interesting stuff.

So ScottyUK and CJ, are you checking burdens and saturation factors for every ammeter installation? I'm not too concerned about about damage to instruments or CTs, or even loss of protection. It's the people exposed to a potentially open CT circuit that would concern me.

I haven't found an ANSI/IEEE/NEMA standard that addresses the ammeter side yet. Maybe the manufacturing standard is simply CYA. I would think that standard would cover both sides of the Atlantic, though.

 

[ScottyUK]

Hi Stevenal,

In answer to your first question, no more so than you are, I bet. I just prefer to keep ammeters out of protective relaying circuits, as it's my opinion that protection circuits are best kept as simple and with as few weak links as possible. Adding ammeters into the loop isn't compatible with that philosophy. The original question was about adding ammeters into the relay circuit, not about using protection-class CTs with ammeters; this thread seems to have changed direction slightly.

As you say, the safety hazard presented by open-circuit CTs is far greater concern than damaging an ammeter.




-----------------------------------

Start each new day with a smile.

Get it over with.

 

[jbartos]

Comment on stevenal (Electrical) Apr 9, 2004 marked ///\\It's the people exposed to a potentially open CT circuit that would concern me.
///The CT secondary may be protected by a voltage limiting device so that the open CT circuit is always safe as far as the CT secondary voltage is concerned.\\\