Line Side Taps- meshing of currents?

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gwr71

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Hi, I have two questions that I hope the experts in this forum can shed light on. First, does Insulation-piercing tap splice connectors act like a CT?
That is, a CT normally reduces the current from a high amperage say 100 amp to a much smaller and hence measurable amperage say 5 amp. CTs can be both for measuring and for overcurrent protection. In the latter the CTs are normally connected to a relay then to a contactor/circuit breaker. When the utility current reaches dangerous levels then the CTs will send a signal to the relay and then the relay to the contactor/cb to open the circuit. With a line side tap these insulators are used and the tap connectors are connected to line of fused disconnect, OCPD. Does this mean that the Insulation-piercing tap splice connectors are really CTS rather than reduce the voltage?
Second, There are now two different current sources passing though the fuses. The utility current at Line and the Inverter ac output at Load. Should this not cause the two current sources to burn themselves out and activate the fuse or Is there a meshing of both currents so that they are safely passing through the fuses?
 
Hi gwr,

Your occuption of "consultant", and your apparent unfamiliarity with the basics of electricity lead me to believe you may not be a professional in the electrical industry. So for now we cannot give you any advice that could be considered instructions for electrical work.
We can answer general questions about electrical installations, basics of electricity, and interpretation of the NEC. Does Nevis enforce the NEC or some other electrical code?

1. An insulation-piercing tap connector acts like any other electrical splice. It allows current to flow from the wire it connects to through the tap conductor. The current flowing has no relation to the current in the main conductor. They are often used to drive a voltage meter or voltage sensing relay and there
must be a fuse in the tap conductor to protect against short circuit or overload.
2. This question is a little harder to figure out what you mean.
The inverter supplies current at the same voltage and frequency as the utility source it is attached to. That current will either flow into the grid, back through the service disconnect, or some combination of the two. Regardless of how this plays out, the current flowing through the service disconnect will be exactly what is required by the load, nothing more, nothing less.
 
An insulation piercing connector is simply a conductive splice. It is electrically equivalent to a large wire-nut.

A CT is a totally different beast which uses magnetic coupling to make the current in one circuit proportional to the current flowing in a completely separate circuit. Ideally there is no power transfer, but instead a proportional current is produced for metering use.

As far as there being two 'sources' on the line side of the fuses, the PV source is designed to synchronize with the grid and just become one of the many generators feeding the grid. If you connect a non synchronized generator to the grid all sorts of ugly ensue, but the inverter is specifically designed to prevent this.

As to which way the power flows with a synchronized PV source, I like this analogy: https://forums.mikeholt.com/threads/question-regarding-where-power-goes.2562979/post-2695041

Jon
 
1. I thank you for the clarification as to the insulator taps not being a CT but just a splice which brings the grid current through the fuse disconnect.
2. In respect to the two current sources passing through the fuses, it means that the two sources mesh together at this point. They are inseparable at this point. I have read that some grid tied inverters have bi-directional circuit breaker like Sol-Ark 12 K so that the inverter can send the inverter ac output through this bi-directional CB rather than the fuse disconnect. If this happens then I may conclude that there are two avenues for the grid tied inverters to feed the grid from the inverter ac output.
3. Since joining this Forum earlier this year I have taken the advice given and I am learning about electricity from online courses but there is a lot to learn. It takes time as it cannot be rushed.
 
Please note that the issue of meshing of currents is very important. As I want to appear smart and not dumb this is why I was confused:
1. Most grid-tied inverters have bi-directional circuit breaker (cbs) and the inverters send the inverter ac output through this cb to the grid, There are 2 CTs on the line side of the meter for the sole purpose of measuring the amount of current being fed to the grid.
2. Therefore there is no need for a fused disconnect as the grid is fed by the bi-directional cb. This is why (in my opinion) the NEC stipulates that solar systems under 10kw don't need a line side tap for overcurrent protection.
3. In solar systems over 10kw and exceeding the 120 % or in some cases 125% allowance, then the NEC stipulates either a line side tap or a load side tap.
There was my confusion. The real purpose of a line side tap is overcurrent protection and not to feed the grid. Thus there must be two current sources at the point of the fused disconnect. Either the insulator taps that splice at the same time function as a CT and reduce the amperage to meet the amperage of the inverter ac output or the full current and voltage( ex 100 amperage )of the grid current flows through the fused disconnect. and meets the inverter output ac. Therefore my heading Line Side Tap-Meshing of Currents?
 
gwr71 said:
Hi, I have two questions that I hope the experts in this forum can shed light on. First, does Insulation-piercing tap splice connectors act like a CT?
That is, a CT normally reduces the current from a high amperage say 100 amp to a much smaller and hence measurable amperage say 5 amp. CTs can be both for measuring and for overcurrent protection. In the latter the CTs are normally connected to a relay then to a contactor/circuit breaker. When the utility current reaches dangerous levels then the CTs will send a signal to the relay and then the relay to the contactor/cb to open the circuit. With a line side tap these insulators are used and the tap connectors are connected to line of fused disconnect, OCPD. Does this mean that the Insulation-piercing tap splice connectors are really CTS rather than reduce the voltage?
Second, There are now two different current sources passing though the fuses. The utility current at Line and the Inverter ac output at Load. Should this not cause the two current sources to burn themselves out and activate the fuse or Is there a meshing of both currents so that they are safely passing through the fuses?
Click to expand...
CTs transform current (CT = current transformer), not voltage. Perhaps that was a typo on your part.

Insulation piercing connectors are not CTs. They do not alter current or voltage.

As for inverters, they have an internal synchroscope that synchronizes the phases with the mains.

And you are correct, meshing of the current, as you say. If the sine waves are not aligned, things blow up. Now an inverter can only approximate a sine wave, but never create a pure one. You can make the resolution as high as you want, but analog waves have infinity variability. This fact is a problem with inverters, and all things connected to an inverter should be rates as such (i.e. motors)

Sent from my BE2028 using Tapatalk
 
gwr71 said:
Please note that the issue of meshing of currents is very important. As I want to appear smart and not dumb this is why I was confused:
1. Most grid-tied inverters have bi-directional circuit breaker (cbs) and the inverters send the inverter ac output through this cb to the grid, There are 2 CTs on the line side of the meter for the sole purpose of measuring the amount of current being fed to the grid.
2. Therefore there is no need for a fused disconnect as the grid is fed by the bi-directional cb. This is why (in my opinion) the NEC stipulates that solar systems under 10kw don't need a line side tap for overcurrent protection.
3. In solar systems over 10kw and exceeding the 120 % or in some cases 125% allowance, then the NEC stipulates either a line side tap or a load side tap.
There was my confusion. The real purpose of a line side tap is overcurrent protection and not to feed the grid. Thus there must be two current sources at the point of the fused disconnect. Either the insulator taps that splice at the same time function as a CT and reduce the amperage to meet the amperage of the inverter ac output or the full current and voltage( ex 100 amperage )of the grid current flows through the fused disconnect. and meets the inverter output ac. Therefore my heading Line Side Tap-Meshing of Currents?
Click to expand...
Man, I hope you're not doing some DIY project. Why are you asking these specific questions? Is this a current installation you're curious about or something you're planning to do? Please don't attempt to make such an installation yourself, hire an electrician.

Sent from my BE2028 using Tapatalk
 
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