I came across an oddity in a ATS. The building has a monitoring system installed by a 3rd party BMS company (with some downright scary work). They installed their own wiring into the ATS and put a CT on one phase only on the normal and emergency inputs, the output is looped through another instrumentation CT and that feeds into a PLC. This is all going away, but I was curious as to what would be the purpose of having a CT on only one phase. Any thoughts?
CT on only one phase -what for?
- Thread starter Todd0x1
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zbang
Senior Member
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- Roughly 5346 miles from Earls Court
Probably the assumption that there's never going to be current in only one of the other phases; probably true if at least some of the loads are delta-connected.
FWIW
We use two CTs for three phase systems and one CT for single phase. But then I'm just a Brit................
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Might have been a simple go/no-go current flow indicator, meaning they were not measuring per se, they were just telling the PLC that current was flowing and that the transfer switch had worked. The PLC may have had a load shedding scheme for taking unnecessary parts of the facility off line if running on a generator.
kwired
Electron manager
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- NE Nebraska
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- EC
For single phase (from a single phase source not two phases and neutral of a wye source) you can route the two lines through the CT in opposite directions and it will measure sum of current of both conductors. POCO's do CT metering of some single phase services this way, and I think many of their single phase self contained meters also measure with this same method.FWIW
We use two CTs for three phase systems and one CT for single phase. But then I'm just a Brit................
Ok that makes sense. There were taps off each phase presumably to measure voltage as well. That part is downright scary, about 6 feet of tffn between the 1200A 480v buss and a 3 pole fuseholder, then the now fused conductors land unaccounced on a terminal strip in a control panel. This was not factory, it was installed by the BMS company. No load shedding, it used to be a telco facility 500kva utility transformer and a now removed 750 kva generator. All the BMS stuff was telemetry for remote site monitoring, and very poorly implemented -ice cube relays sitting loose in 4S boxes, stuff like that.
On this one they put ring terminals on the wire and placed those under the heads of the bolts holding the lugs to the buss. I don't think thats great either since the low compressive strength of the small copper ring terminal could affect torque no?
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