PM5300 vs 5560 - 3-wire RS485 wiring differences?

sesa10844_bridg · ‎2014-09-02

In reviewing the installation guides for the PM5000 line of meters, there's a slight difference in the wiring the PM5560 and PM5300 to downstream 2-wire devices. The PM5560 shows that you should wire the shield connection on the 2-wire devices to the 0V terminal on the 5560, but to the shield terminal on the 5300. Is this correct?

From the PM5560 installation guide:

From the PM5300 installation guide:

James_Clark · ‎2014-09-24

Historically, 2-wire RS485 connections were wired with a single twisted pair and a shield conductor. The markings on the devices were +, -, shield or L1, L2, shield. Some devices from Industry BU use the provided example of D1, D0, shield. The shield in most cases was actually the RS485 common or the 0V connection. On new devices there are 4 connections: +, -, shield, 0V (common). Electrically, when mixing old and new devices the shield on old devices should be connected to the 0V connection on new devices. This 0V connection should then be connected to ground at the source or start of the daisy-chain. This configuration is important when multiple devices are installed on a daisy-chain - especially if there is a mixture of older and newer devices.

The newer devices with 4 terminals for 2-wire RS485 provide a landing point for the shield that is not electrically connected inside the device. It is purely a landing point to simplify providing a continuous shield. This shield connection should then be connected to ground at a single point, ideally at the source or beginning of the daisy-chain.

NOTE: For some ungrounded metering / monitoring devices, it is recommended that the 0V (common) conductor be connected to ground through a 500 ohm resistor to prevent leakage current. As with the shield connection, the 0V conductor should be connected to ground at a single point, ideally at the source.

See Answer In Context

VictorMarincus · ‎2014-09-03

Yes, it is correct if we assume that there's only one PM5300 in the picture. Please refer to this discussion - you might find it useful:

JamesAZZO · ‎2014-09-16

We have the same question.

sesa207393_brid · ‎2014-09-17

Hello Victor,

Thanks for your great explanation, very helpful!

But for PM5300, if there are some devices do not have the C terminal in my RS-485 network, I think the best choice is that using the bare wire in the RS-485 cable to connect the C terminal from the PM5300 meters to the shield terminal on the devices that do not have the C terminal. But in the PM5300 user guide, the suggestion is opposite, is it special?

VictorMarincus · ‎2014-09-18

, could you please get in touch with someone who can clarify why the PM5560 installation guide mentions that the Common terminal (of a PM5560) should be connected to the shield terminal (of several PM800 - let's say) - See the second situation in Daniel's post above ("From the PM5560 installation guide:")? To me it looks like there is a mistake there.

James_Clark · ‎2014-09-24

Historically, 2-wire RS485 connections were wired with a single twisted pair and a shield conductor. The markings on the devices were +, -, shield or L1, L2, shield. Some devices from Industry BU use the provided example of D1, D0, shield. The shield in most cases was actually the RS485 common or the 0V connection. On new devices there are 4 connections: +, -, shield, 0V (common). Electrically, when mixing old and new devices the shield on old devices should be connected to the 0V connection on new devices. This 0V connection should then be connected to ground at the source or start of the daisy-chain. This configuration is important when multiple devices are installed on a daisy-chain - especially if there is a mixture of older and newer devices.

The newer devices with 4 terminals for 2-wire RS485 provide a landing point for the shield that is not electrically connected inside the device. It is purely a landing point to simplify providing a continuous shield. This shield connection should then be connected to ground at a single point, ideally at the source or beginning of the daisy-chain.

NOTE: For some ungrounded metering / monitoring devices, it is recommended that the 0V (common) conductor be connected to ground through a 500 ohm resistor to prevent leakage current. As with the shield connection, the 0V conductor should be connected to ground at a single point, ideally at the source.

VictorMarincus · ‎2014-09-24

Thank you James for the clarification

Daniel_Brandao · ‎2014-09-24

We will change the documentation so that PM53xx is consistent with the PM55xx. Thanks for bringing that to our attention.

sesa205726_brid · ‎2014-09-25

Hi James,

I work in Italy to the technical support center from 3 years and I always worked with Power Logic product range and, in general, with all modbus device (Acti9, Power Logic, Compact /Masterpact, etc…).

I followed the evolution of the product range and also the serial line communication.

New products have a new RS-485 connection (4 connection points) and the monitoring/meter device actually sell yet (EGX100/300, Compact NSX, etc…) have 3 connection points.

Many customers ask me how connect old and new devices together and about this question I read many posts but, I am honest, nobody was been persuasive. I read many thoughts where are used the words “Should be” instead of “Must be” and this shows the existence of a widespread uncertainty.

Now, in your post also read the words “should be” and a news for me:

“The newer devices with 4 connection points for 2-wire RS485 provide a landing point for the shield that is not electrically connected inside the device. It is purely a landing point to simplify providing a continuous shield. This shield connection should then be connected to ground at a single point, ideally at the source or beginning of the daisy-chain.”

I don’t know where it is written it. Please, can you send the document reference? Also, what are the products that have this characteristic?

Reading your post I also found that the cable shield should be connected to 0V (common). My question is: if I have shield continuity through this connection, why is it provided also a landing point?

I would like, as producers of these product, that we have a standard to follow and a common thought.

I know, how written by in the post [DEAD LINK /message/13585#13585]https://powersolutionscommunity.schneider-electric.com/message/13585#13585, the function of 0V common point.

I want take as standard the connection schema of PM55xx where the landing point is left free. But the question is: if I can leave free this point, what is its utility?

I would like receive an official answer from someone of the development center, but I don’t know how I can reach them. I don’t if you can help me to this scope.

Sorry for my strong comment but I want know exactly what I must say to the customer avoiding subsequent communication problems.

I want share this post also with

@Thank you

BR

James_Clark · ‎2014-09-26

Salvatore,

Excellent questions.

Schneider Electric established a new standard for communicating devices with RS485

communication ports a few years ago. This new standard requires 4 conductors for 2-wire

networks and 6 wires for 4-wire networks. The new standard requires a conductor for 0V

(common) and a landing point for the cable shield.

I do not have a complete list of all the devices that have implemented this new standard. I

know the PM5000 series and the PM8000 series power meters both utilize this approach.

Older devices such as the PM700 series and PM800 series power meters utilized only 3 conductors

for 2-wire RS485. On these older devices, the connection labeled "shield" is actually the 0V

(common). Therefore, when wiring these older devices along with newer devices, the shield

conductor of the cable should be landed on the 0V terminal of the newer device. This will

leave the "shield" connection on the newer devices not connected which is fine.

When wiring a daisy-chain of newer devices implementing the 4 conductor connection, a conductor

should be dedicated to the 0V connection and a separate conductor for the shield conductor.

The shield should then be connected to ground at one point at the daisy-chain source.

There are many references for RS485 wiring best practices. One really good reference is B&B

Electronics web site. You can also refer to installation guides from many Schneider Electric

devices. You will see different recommendations due to the age of the product and the

application. For example: many protection devices will recommend a non-continuous shield

where the shield is connected to ground at each device.

Regards,

James

VictorMarincus · ‎2014-09-26

This is something for the context - cables used.

aaron_azzo_com_ · ‎2014-10-09

James,

Can you please confirm your note above ("NOTE: For some ungrounded metering / monitoring devices, it is recommended that the 0V (common) conductor be connected to ground through a 500 ohm resistor to prevent leakage current. As with the shield connection, the 0V conductor should be connected to ground at a single point, ideally at the source) applies to the EGX100/300's as well? Just want to be sure this would not cause any internal issues to the EGX.

Thanks James.

James_Clark · ‎2014-10-10

Aaron,

We are not aware of any issue this would cause with the EGX. This is the wiring recommendation

that was recently released for High Density Metering Enclosures and should be able to be

applied to any daisy-chain of ungrounded meters.

Regards,

James

dborle · ‎2014-10-15

Hi James,

What happens with devices were we have only 2 conductors like the DM6200/PM1200?

Thanks

Daniel