Questions & Answers > 3 phase 4 wire delta question

I have a question with an installation I have found. It is in Book 2 of the Bible on page 187. It is an open bank, 3 phase 4 wire delta with 2 equal ratio ct's. There is a form 5S solid state meter there and when looking at the drawing you say that a Fm 5S solid state meter will not measure correctly and myself and the journeyman are wondering why not. We are going to replace it with a Fm 45S but would like to know why a Fm 5S solid state is not the meter for this application.
Bud Scott

March 13, 2012 | Unregistered CommenterBud Scott

Thanks for your question. I am happy to provide you with an answer to it.
First, I would like to start with some background information and define the answer from there.

1) In all multi-element solid state meters (no matter what form number) the non-polarity side of all the meter potential coils has a tie to one common point. If you look at a meter form drawing for any multi-element solid state meter you will find this to be true (you can find these types of drawings in the Meterman’s Bible). This common tie point for solid state meter potential coil non-polarity connections is how they must be connected for the auto-ranging feature to work and that is why a solid state form 5S meter can't be used on 4 wire Delta. It will cause a billing error.

2) My experience with this comes from the early 1990's when the company I was working for starting buying and installing solid state ABB Alpha meters on all of the commercial installations. Up until the time when we began changing over to solid state meters, we would install two CTs and a form 5S electro-mechanical meter on most of the 4 wire Delta installations. The form 5S electro-mechanical meter is a full 2 element meter with 2 current coils and 2 potential coils. In the form 5S electro-mechanical meter each of the potential coils(2) have individual connection points to the non-polarity sides of them. With this type of meter form set-up we could meter 4 wire Delta by energizing the left hand side element potential coil at 240 volts (phase to phase) and the right hand side element potential coil at 208 volts (phase to neutral) and by so doing cheat Blondel’s Theorem. (According to Blondel, it would require a 3 element meter on a 4 wire metered circuit).

3) The form 5S solid state meter (when it was made) was also a full 2 element meter (2 current coils and 2 potential coils) and when we started installing these meters into the same 4 wire Delta installations we found that they were creating a billing error for the customer. Because the non-polarity sides of each of the potential coils(2) in the form 5S solid state meter has a common tie point it made it so that when this meter was installed into the 4 wire Delta installations both potential coils (non-polarities) ended up being tied to neutral(ground). Doing it this way caused the following to occur:
• The voltage across the left hand side potential coil was now connected from phase to neutral at 120 volts instead of phase to phase at 240 volts (like the electro-mechanical meter that was used previously).
• The voltage across the right hand side potential coil was correct at 208 volts and this is because it was always connected phase to neutral.
• The common tie for each of the meter potential coils was something that was discovered by the meter department after the problem with 5S solid state meters not being able to correctly meter a 3 phase 4 wire Delta installation with 2 CTs (CTs the same size or different sizes).

Once this error was discovered (and a new meter form had been created) we had to go to each 4 wire Delta installation where a form 5S solid state meter had been installed and replace it with the newly designed ABB Alpha 35S meter that was designed to take the place of the 5S solid state meter which, as it turns out, had been poorly designed for use with this style of 4 wire Delta installation.

A couple of notes here:
The original form 5S solid state meter will work correctly on 4 wire Delta IF secondary voltage transformers are installed (because the connection of the secondary VTs has a common point for both of the secondary voltages). The solid state 5S will work fine on 3 wire Delta (with or without voltage transformers [The 35S or 45S will also work]).

For a form 5S solid state meter to correctly meter a 3 phase 4 wire Delta service: The left hand potential coil has to have the correct phase to phase voltage across it. With the left hand potential coil energized at only 120 volts (phase to neutral) rather than 240 volts (phase to phase) that portion of the meter will see approximately 50% of the customer's usage that should be metered by that element.
• The left hand side element measures all of the customer's 120 volt loads, some portion (maybe all) of the single phase 240 volt loads and ½ of the three phase loads.
• The right hand side element will meter correctly and it might be metering some of the single phase 240 volt loads along with ½ of the 3 phase load.
• The total billing error is fully dependant on the amount of 120 volt, 240 volt single phase load along with the amount of 3 phase loads and how and when they are used.

The best way to determine the overall error is to correct the installation by installing a form 35S or a 45S meter, which will correctly meter this installation and the connected loads. Once the new meter is in place you can find the billing error of each customer by taking the reads for a couple of billing cycles and comparing these reads against usage history (previous billings).

I have emailed you two pages of information regarding the voltages that are across each of the potential coils for a form 5S electro-mechanical meter, an originally designed form 5S meter solid state meter, and a redesigned 5S to become the 35S or 45S solid state meters. These diagrams show the potential coil voltages for each connected form number installed into the 8 terminal socket as shown in Drawing 73B on pages 187 and 188 in the Meterman’s Bible. This described metering error is fully a function of incorrect voltage across the left element potential coil. There is no problem with the CTs or the current coils in the meter so this is not shown on these diagrams in hopes to reduce any confusion regarding this. If you have problems opening the file please let me know. For any further questions or concerns, regarding my answer or the diagrams, please contact me by phone and we can talk about them (503) 901-6132

Thanks and Be Safe All Ways,

PS: If anyone else is interested in obtaining these diagrams for reference, please contact me via e-mail at

March 13, 2012 | Unregistered CommenterBrad