Shorting Links for all CT? 2

[czping]

Hi, I understanding that a CT secondary shall not be open circuited due to the built up of high voltage across the CT secondary terminal. However, does this means that all CT secondary shall have shorting links?

We have a CT feeding into a earth leakage relay (ELR) and when the ELR is faulty, we will just change it without shorting the terminals cos there is no provision for it. Is this dangerous?

Thanks!

 

[scottf]

You can just use clip leads to short the secondary, if no shorting links are provided.

Since this is for earth fault detection, there is probably not much primary current flowing, which means that the secondary open circuit voltage shouldn't be too high, but you have no real way to confirm that.

If it was me, I would short the secondary before removing the relay.

 

[tomatge]

A related question that has often bothered me is this ...

If one were to open circuit a CT in service, actually what would be the induced voltage, and the consequent danger. My colleagues all talk about "blowing up" CTs and killing people and the like, but can anybody give a simple rule of thumb as to how dangerous this would be.

Thanks in advance.

 

[davidbeach]

If the CT didn't saturatate, the open circuit voltage would be high enough to allow the secondary current to flow across the insulation between the terminals. Core saturation limits the open circuit voltage, but it can be enough to cause serious trouble. A lot depends on the type of CT and the primary current.

 

[scottf]

As Davidbeach says, it very much depends on the type of CT.

The open-circuir voltage is a function of the following parameters:

-No. of secondary turns
-Cross-sectional area of the core
-Core material
-Primary current

In terms of magnitude of voltage, it could be anywhere from a few hundred voltage for small metering cores, to 50 kVp or more for large protection cores. Note that the voltage that appears is not a sinusoidal wave form and the danger comes from the very high peak voltages. A common mistake is to try to measure the voltage as an rms and the rms voltage appears relatively low.

 

[Bung]

The "blow-ups" referred to are often in oil-filled CTs where the very high peaky voltages mentioned to by scottf have punctured the insulation and established a current path through the broken down insulation, started some heating, boiled the oil, ignited the vapour and kablooey. Things can go pear-shaped very quickly.

As a safety issue, the golden rule should always be NEVER, ever, leave a CT open-circuit.


Bung
Life is non-linear...

 

[jbartos]

Suggestion: Only certain types of CTs have high secondary voltages when the secondary winding is open.
E.g. Wound CT 5A:5A will not be posing any voltage danger on its secondary if the primary voltage is low.

 

[edison123]

jb,

what has primary voltage got to do anything with secondary open circuit voltage of a CT ?

 

[jbartos]

Suggestion to the previous posting: Read the original posting more carefully and notice the transformer turn ratio=5A/5A=1=Primary volts/Secondary Volts
If the primary side happens to have high voltage then, the secondary side would have high voltage too, under an assumption of certain VA size of CT, e.g. 100VA. The primary side of 100V would imply the secondary side of 100V. If the CT VA is less e.g. 50VA, then the voltages will be one half.

 

[edison123]

jb,

apparently, ct is defined differently in the books you studied. Common definition of Ct's is their primaries are in series with the load. So, you can explain how the voltage (across the terminals) comes into scene in re open ckt secondary voltage.

 

[alehman]

Also see thread238-60103

 

[scottf]

As is said previously, the primary voltage has nothing to do with the open circuit voltage on the secondary. The voltage developed across the secondary when open circuited is due to the primary current being shunted through the magnetizing branch. Since the primary current is essentially forced through the magnetizing branch, the voltage built up on the secondary is a function of the magnetizing branch impedance.

 

[jbartos]

Suggestion: The previous posting points to a very informative thread. Essentially, the magnetization branch of CT is the culprit causing high secondary voltages.
Visit

http://www.tpub.com/content/doe/h1011v4/css/h1011v4_63.htm

etc. for more info

 

[twosockets]

To all,

There is a great deal of misinformation floating around out there about open CT secondaries. The following are a couple of examples:

1. A lineman actually called me a liar while I was teaching a metering class because I warned about the dangerously high voltages that could be present on an open CT secondary. Having heard me give this lecture previously, he used a standard RMS voltmeter to measure the voltage on a large primary CT that he had deliberately opened the secondary on with a heavy primary current. He said that he measured a voltage of less than 100 volts and that is less dangerous than a standard wall outlet! After I asked him what kind of personal protective equipment he was wearing while performing his measurement (none, not even gloves), I told him that his voltmeter was incapable of measuring the > 3000 peak-to-peak voltage that was probably present. He turned a shade of green!

2. One statewide safety committee actually told its captive members that transformer rated metering installations with VTs should not be considered for 480 volt applications! They told its members that a malfunctioning CT shorting device failing to operate would cause a 600-volt rated donut CT to explode like a hand grenade! They required that the service be deenergized to change out the meter. This incredible misinformation was taught for years until I blew the whistle. As you can imagine, it was difficult to teach common sense practices without making it appear that you were advocating unshorting CT secondaries!

Rather than explain why it is desirable to short the CT secondaries on small donut CTs and extremely important to short the secondaries of large or primary rated CTs, many utilities and contractors simply dictate that ALL CT secondaries MUST be shorted before removing the connected device from the circuit.

 

[ScottyUK]

twosockets,

I had a brown-trouser experience when a cable pulled free from its crimped lug on a CT secondary circuit. The CT was an 18000/5 Class X on a generator transformer. The hissing sound of the arc was scary, I reckon it was approx. 15mm long and burnt a hole in the terminal and the DIN rail it was mounted on.

I have no recollection of vaulting over a guard rail 8 feet above grade level to get away from it. A colleague told me about that afterwards!

Moral of this tale is treat CTs with a *lot* of respect. Treat big ones with even more respect.

 

[jbartos]

Suggestion: The CT open secondary has been the topic of one of reports pertaining to fire hazards, whether or not a spark due to open CT secondary circuit may ignite material and cause a fire in a generating station.