CT's

mazthespark

Hi,
i have a Question relating to the use of one set of ct's with 2 KWH meters.

I have an application where i have two users of the one supply (at different times) I plan on using 2 seperate kwh meters and depending which user is present one or the other KWh meter will be on so that when esb bill comes in the bill cant be split accordingly.
Can i use the one set of cts to acheive this and loop them to each Kwh meter( only one will ever be on) or do i use 2 sets of cts? is there any problems that can occur if current is flowing through cts and they are not connected to anything eg the kwh meter is off.
both users make use of the same equipment when the system is in use so 2 seperate panels or whatever is not an option i just need a means of measuring what each use.
any one have any suggestions? i have used cts before but never in this kind of application.

Bruthal

What about 2 key switches, who ever is using the equipment operates their switch, which can have a contactor setup swapping the equipment to their meter.

Once one key is still in, the other wont do anything, using electrical interlock. If last user forgets key, bad luck:)

Why the CTs, a large load, or because you felt it would facilitate this setup?

They could still be used as above, with one ct switching between the 2 meters. CTs need to be closed circuit anyway when around a conductor.

mazthespark

the key switches are used to energise the kwh meters. the reason for not using contactors is that the load is quite large up to 60kw 3 phase load, hence the reason for the cts method.
so i could use just one set of cts then?

Bruthal

Is the problem just finding a way of using one CT for 2 meters, or metering 2 separate users?

As in, could a single key switch be used to swap the CT from one meter to another? Or will the load need to be disconnected, so one user can not use the other users meter?

Splitting the supply line and a CT on each line to each meter, and electrically interlocked contactors swapping the load from one line to the other via key switches could be another method to separately meter according to each user.

mazthespark

needing to meter two seperate users is the issue. its a retro fit. only one user will be using the equipment at a time the other will not be able to. this will be done by 2 seperate key switches. the reason for not using contactors to split the load is the large cost of 60kw contactors!!
using two sets of cts is not a problem i was just curios as to whether or not runnin load through disconnected ct's (ie the set on the de energised kwh meter) would cause them or the equipment any damage? hence why i thought of using one set and switching them between the appropriate kwh meter depending on the user.

Bruthal

A ct needs to be kept closed circuit the whole time. They are sort of like a standard transformer but almost like an unlimited ratio primary to secondary, so if the CT is left open circuited while current is flowing through the enclosed conductor, the voltage on its terminals can reach dangerous levels.

Without a contactor setup to switch the load along with the metering, one user can come along and just use the equipment without switching meters. But thats probably not a problem here anyway by the sound of it. 60kw 3 phase is about 86 amps per phase at parity anyway.

Using 2 CTs, relays can simply have one DP contact, switching off the meter, and shorting the CT as the relay changes over.

Or one CT can be used, with 2 DP relay contacts swapping the CT connections from one meter to the other.

Another possible method is to have 2 CTs, one hard wired to each meter. When the key switch is switched, it causes a relay to short one CT, stopping its meter, and un-shorting the other, bringing its meter on. Whether shorting a CT at its terminals will completely stop a meter connected to it, im not certain. One experiment I never tried.

A single 2 way key switch can be used anyway, with each user having a key to change from one meter to the other.

dingding

http://openenergymonitor.org/emon/sites/default/files/meters-incomers.png

It has been a while since I worked with meters and I don't know if this would work, it should in theory.

If you look at the terminal block C in the image linked above, you will see where the current goes in and out of the meter there is a little link on the top. This supplies the voltage part to the meter. If you open this link on all the phases the meter will stop. If you wire the 2 meters in series so the current os flowing through both meters and just supply voltage via a contactor to the meter you want to use.

You could use 2 contractors and and interlock them controlling them with a set of key switches. The contractors could be small as they would only be supplying the energising the voltage to the meters.

Someone with more recent experience metering with the ESB Networks might be able to advise.

Another idea

http://www.publishing-expert.com/b2b/pics/Infrared_Interface_Single_Phase_Multi_Rate_Watt_Hour_Meter.jpg If you put in a day-night meter and just give each client one of the sets of dials and they can switchbetween dials with the key.

ted1

What type load will be used, this will determine the amount you can spend.

I'd look at using a meter with a pulse output and bring in it to a datalogger and get quarterly hour data, that way its very easy to split the costs.

If it's just this one load, you could look at using an iskeameco p2g logger and setting up an account with smsmetering. Meter is about 160gbp and service is about 12gbp, then its just the cost of a daily text.

Another way is to use the pulse to drive 2 separate counter units. Run these off one timer connected to a relay so that one counter is always connected.

mazthespark

What i have ended up do ing is using 2 kwh meters and one set of cts.
the cts are ran through a relay and if one user energises the system the relay will energise. if the other user energises the system the relay will stay de energised. thus giving me the switching between 2 kwh meters.

Steve

P=I*V

All you need do is route the current (I) through both meters permanently and only apply V to the one you want to read.

Open circuiting CT's is bad, even for the briefest of times and will cause arcing and premature death of the switching device.

dingding

I talk about this in Post 7.

Also if the switching device was to fail it could be a dangerous situation, as you would have high voltages across the fault. And if there was a failure of the switching device you would not be recording all the power used correctly.

adrian92

I do not know exactly what you are doing but would make two points:

1. Ensure, at all costs that your set up does not cause a CT to be in an open-circuit situation, on the secondary side. (really bad things can happen, in this event)

2. Normally only one supply to a building- sub-metering , perhaps would be better to divide costs .

Must be a big kW load if CT's are involved.