CT wiring - Can a #10AWG wire come loose from a standard terminal block over time? 2

[bdn2004]

Recently I posed a question on this site about the size of CT wiring. The links provided a good basis that #10AWG wire should be used.

I was talking to a Contractor and he said the problem is the terminations aren't good with that size wire. He didn't go into details but I'm assuming he meant that it's hard to tighten them down so they don't come loose over time? He recommended a ring lug and a special clamp down type terminal.

We have a project going on right now that has some CT wiring but no special terminals like these are provided by the Vendor. The terminals provided are rated to accept a #10 wire but I wonder if we should do something special here like what the Contractor suggests? I know these CT circuits can be dangerous.

 

[oldfieldguy]

I have, over the years, installed or supervised the installation of a large number of current circuits and have never had difficulty with terminals coming loose over time.

[ul]
[li]Buy terminals from a reputable manufacturer.[/li]
[li][/li]
[li]Use a properly maintained complete-cycle crimping tool that is designed for the termination to be used..[/li]
[li][/li]
[li]Have an installer that knows and cares about his work.[/li]
[/ul]

I concur with the use of ring lugs on current circuits, but as with any installation, the quality and ultimate reliability rests heavily on the work of the installer.

I've seen some pretty crappy terminals form the bin of a 'big-box' store. Don't use those. I've seen 'technicians' using the entirely WRONG crimping tool.. Neither of those will get you where you need to be.

old field guy

 

[dpc]

At least in the US, it would be consider poor workmanship to use anything except ring lugs for CT secondary wiring, and would be unacceptable in most situations. This is generally covered in the specifications. Shorting-type strap-screw terminal blocks are generally used for all CT wiring.

I agree with old field guy, even when using ring lugs, poor workmanship and low quality lugs are still a concern. When properly installed, a good quality ring lug should be reliable.

I'm curious as to what is used in the rest of the world?

 

[scottf]

dpc-

I agree with your comments on substation-type / relaying-type installations.

However, for metering type CTs used for low-side 600V metering and even for MV CTs used for primary metering and/or protection, it's quite common/normal to use the compression-type terminals provided with the CTs. The CTs we make actually have terminals that can be used as compression and can accept ring-crimp terminations, but I can't think of any installs I've ever seen using crimp connections.

 

[3I0]

Typical installations here would use good quality ring (not fork) connectors and screw-down shorting terminal blocks.

The only exception to this I have seen are designs that are built to older Ontario Hydro standards where screw connection terminal blocks are used on bare wire with two clamping points per wire. Example would be Phoenix Contact URK-ND 2 terminals. These can also be equipped with shorting links, etc.

 

[FreddyNurk]

After some consideration (and a not insignificant amount of resistance from some staff) we're now starting to use the 'Cage Clamp' style of links for CTs. These come complete with integrated shorting links so CTs can be shorted without shorting leads or other arrangement, and have touchproof banana plug sockets for secondary injection testing.

I've not seen any of our protection CT circuits with ring terminals, though some of our equipment is sold with ring terminal connections for CTs. All of our stuff is LV (415V p-p) though. As the CTs (protection at least) have protruding studs, we use ring terminals on them though.

oldfieldguy's right though, it doesn't matter what the terminal or crimp arrangement, if you've got a half-arsed installation crew, you're going to have issues.

 

[submonkey]

Where I work, the rules for CT terminations are:

- Captive connections made using ring-lugs with bolt-type terminals.
- The ring lugs are formed from tube stock (split barrel lugs not permitted).
- Typical terminal stud size is 5mm.
- Preferred terminal material is stainless steel.

I've never seen a problem with the above when combined with proper workmanship.

 

[ScottyUK]

We generally specify the Weidmuller RSF series of terminals for CT connections. The RSF series are a standard cage clamp type similar to the common SAK series, but which incorporates a fairly powerful spring which closes the cage onto the lug. When used with lipped flat blades the lip (or hook) engages withe a recess in the terminal which makes it virtually impossible to pull a lug out even if the clamping screw is loose or is completely missing.

 

[stevenal]

Good topic, since we've just had an incident involving a poor crimp on a ring terminal.
Do you use insulated or non-insulated terminals?
Are the formed tube terminals Submonkey mentioned available in insulated versions? If so, any brands to recommend? Web links are even better.

Why stainless? Stainless has higher resistivity than copper.

 

[Sn00ze]

This post might come to you a bit late but this is what we use here by Standard

cheers!

 

[FreddyNurk]

Interesting to see the variations, I certainly wasn't aware of those Weidmuller units, and other parts of our business specify the basic SAK units as a standard. We use the Wago 281 series CT link units, though we do carry out calculations to ensure that even under fault conditions the terminals don't carry more than 30A.

What do others do for CT shorting requirements under test or access?

 

[submonkey]

Stevenal,

Refer:

http://www.gvk.com.au/pdf/utiliux.pdf

(picture on 61st page).

We use Utilux 'H3820' terminals (picture on 61st page (pg 60))
Lugs are type 'H2037' or similar (picture on 38th page (pg 37))

The 'H3820' terminals mount on G-rails. I really wish Utilux
would make a DIN-rail version of this excellent terminal.

Lugs formed from tube stock are available with insulated barrels.

Stainless steel is preferred because:
- It doesn't corrode, even after a long time.
- It is very tough and hard to damage (much harder than copper).
- It does not suffer from brittle fracture (like brass).

The conductivity of stainless is more than adequate when one
considers the length and cross-sectional area of the terminal.

I dislike terminals with clamping screws. Almost without fail,
someone will use the wrong size of screwdriver and damage the head.
Terminals with studs and nuts can only be tightened with a nutdriver
and always seem to remain in good condition.

 

[ScottyUK]

Weidmuller's RSF 2 disconnect terminal, showing the internal spring mechanism.

http://catalog.weidmueller.com/catalog/Start.do?localeId=en&ObjectID=0623320000

The mating lug which locks into the terminal

http://www.jst.co.uk/productSeries.php?pid=329&cat=105

 

[FreddyNurk]

ScottyUK, if you'll excuse the possibly ignorant question; how do you get the wire out again? Or do you need to ensure that the terminal is correctly tightened first, and the spring ensures that the terminal stays tight after installation?

Most of our problems arise from installation issues, terminals not tightened to start with, hence the migration away from them.