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 Post subject: Using switches to connect CTs to 1240
PostPosted: Fri Jul 15, 2011 9:29 am 

Joined: Sat Jul 17, 2010 10:32 pm
Posts: 9
I have several circuits I need to monitor continuously and a dozen or so that I might want to look at occasionally. Is there any reason I can't put CTs on the "occasional" circuits and use a bank of switches to connect one or more of them to a single 1240 channel as needed as long as I don't exceed the amp total for the channel? This way I would have the capability of monitoring maybe a dozen circuits with two channels of on 1240 instead of buying additional 1240s and continuously collecting data on circuits I'm normally not interested in.

If this is practical, would there be any special requirements for the switches in terms of voltage and/or amperage capacity? Would single pole be sufficient or do I need double pole to disconnect both CT wires?

Thanks,

Mike


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 Post subject: Re: Using switches to connect CTs to 1240
PostPosted: Tue Jul 26, 2011 6:28 pm 

Joined: Tue Jul 26, 2011 6:14 pm
Posts: 1
There is a very good reason you CANNOT put your CTs on switches. A CT must never be disconnected while current is flowing on the primary. Here's a small excerpt from Wikipedia (http://en.wikipedia.org/wiki/Current_transformer):
Quote:
Safety precautions
Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary, as the transformer secondary will attempt to continue driving current across the effectively infinite impedance. This will produce a high voltage across the open secondary (into the range of several kilovolts in some cases), which may cause arcing. The high voltage produced will compromise operator and equipment safety and permanently affect the accuracy of the transformer.


Hopefully I've responded soon enough and nothing has melted or been destroyed yet :shock:
If you need to monitor more circuits your best bet is additional meters.
-BuB


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 Post subject: Re: Using switches to connect CTs to 1240
PostPosted: Tue Jul 26, 2011 7:01 pm 

Joined: Fri Jun 04, 2010 11:04 pm
Posts: 61
Location: Central Florida, USA
Brultech's CTs are safe to run without a load.
paulb wrote:
Our SPLIT-200, SPLIT-100 and SPLIT-60 CTs all have internal burden resistor and poses no problems if left unterminated under load.

Our Micro-40 and Micro-80 CTs have no termination and will only generate up to approx 14Vdc (low current) if measuring 20A-40A.

It is true that some CTs can generate hundreds or thousands of volts if left unterminated and possibly destroy themselves due to hi-voltage arcing between windings. This does not apply to the CTs we use. Since the magnetic core of our micro CT is so small, it will saturate prematurely with no burden resistor, limiting the amount of voltage that can be generated.

It is recommended to keep the CT leads shorted together if not terminated. Also the breaker should be turned off when terminating the leads to the ECM-1240.
from the thread "CT Continuity Test".
viewtopic.php?f=7&t=192&p=744&hilit=burden&sid=6489d9205f68f58e74a36a4669a5d540#p744

So, while I think that electrically speaking, switching CTs is safe, I don't think it is practical. You would need to worry about the ECM settings and it would make a mess of your database. It seems to me that it would only be useful/interesting in a fairly short time span. When you look at data from 3 months ago, how will you know what circuit was being measured?

_________________
~Wayne in Florida


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 Post subject: Re: Using switches to connect CTs to 1240
PostPosted: Wed Jul 27, 2011 12:05 pm 

Joined: Sat Jul 17, 2010 10:32 pm
Posts: 9
Wayne,

Thanks for the info. It sounds like it's possible but may not be wise. DPDT switches could handle going from shorted to connected to the 1240. I haven't read the CT settings instructions yet, but I assumed that multiple micro-40s in parallel would use the same setting as a single micro-40.

The switched circuit channels are intended for real time analysis and I didn't intend to store that data. The "permanent" channels will of course be stored.

The more I think about it, the more it looks like the best solution would be to find algorithms that will allow me to look at data from multiple circuits and recognise individual loads since even if I had a CT on each circuit, I would still have multiple loads on each circuit.

I guess I should have mentioned that I've been running home grown software with my 1220 for 3 years now. The new 1240 is being added primarily to get more accurate AC output data for my 4860W PV system, to get well pump usage separated out, and maybe to track my pulse output water meter.

Mike


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