Industrial Control Panel Grounding - Page 1
Forum section: electronics / projects

author: Bootalito
author_rank: Unknown
timestamp: December 12, 2017, 07:05:13 pm
content: I'm a control systems engineer(chemical Engineer by degree), and part of my duties include designing industrial control panels. As I've dived into the world of electronics and electrical engineering (I should have been an electrical engineer....) I'm questioning some of the design rules that I've always taken for granted and I wanted some feedback, hopefully from other industrial/automation/manufacturing/control systems engineers who have experience doing control panel design.1. As a standard we tie 24V negative to ground to set the ground potential. This assists with troubleshooting whereas you can just place your black multimeter probe on any metal part and take readings. However this seems to defeat the purpose of a galvanically isolated SMPS. I'm thinking of a scenario where someone accidentally touches 24V to a conduit or enclosure and now you have a path back to 24V- through the building ground. Not a huge dead because most likely there is like a 2Amp fast acting fuse that will blow. But if it was a truely floating power supply this prevent this, right? It can introduce potential ground loop issues on analog control signals which are a nightmare to troubleshoot. I'm thinking that at most one should tie all of the DC power supplies in a panel together to set the same reference, but only if you REALLY need to for some reason.2. We don't fuse the output of a 24V power supply with a fuse rated to its max current. Power is immediately distributed into a row of fuses that then go out to power individual devices requiring 24V power. We will always size the power supply so that it won't ever exceed its rating. However it seems like a miss to NOT immediately fuse the output of a 10A power supply with a 12A fuse(or something), THEN distribute the power to the individual branch circuits. This is what we do with the incoming 120V AC which typically powers the panel. We first go into a circuit breaker, then immediately go into fuses for each 120V branch circuit. Why wouldn't we do the same for 24V power?3. I don't really see the point in having an e-stop reset momentary pushbutton. I mean the ESTOP button is already a push-pull style button, requiring human interaction to pull it out which is, why make someone pull it out, then push a reset button?

author: Someone
author_rank: Super Contributor
timestamp: December 12, 2017, 08:39:51 pm
content: Quote from: Bootalito on December 12, 2017, 07:05:13 pmIn a completely ideal world all the devices would be electrically isolated and you could freely reference the 0V connections as needed. But in the real world all sorts of different parts of the system will have their 0V and chassis connected together, some of those will need to be connected to earth for safety or connected to shields for EMI/RFI/EMC. So you now have 3 requirements competing for the same common 0V/ground/shield connection. In a very high end system you might run 0V busses, as distinct from the safety earth, which is distinct from the single phase neutral, and possibly separated again with special high or low noise busses for groups of devices.Designing all this while still meeting electrical code/regulations can be challenging, more so if there is only limited space in an existing system to fit the work into.Quote from: Bootalito on December 12, 2017, 07:05:13 pmYou can add more fuses, its mostly a matter of style. But each fuse is another failure point and if the power supplies can safely drive a short circuit (most can these days) then all you are protecting is the small fire risk of that wire section creating a matched impedance load and putting all its power into the failure. Again refer to local regulations/codes as power supplies up to several hundred watts may not be considered a fire risk and can be left unprotected.

author: ajb
author_rank: Super Contributor
timestamp: December 12, 2017, 11:28:29 pm
content: Quote from: Bootalito on December 12, 2017, 07:05:13 pmA couple of reasons:- It means the equipment can't immediately go into some sort of active mode if it's powered on with the E-stop button released. Instead, someone has to deliberately push the reset button (IE, be standing right next to the machine, ready to shut it down again if something goes wrong) before the equipment will do anything- If the E-stop loop is interrupted for instance due to a wiring fault, the machine won't immediately activate when that wiring fault is corrected (perhaps while some technician is in harms way looking for a break in the E-stop wiring).- Any temporary interruption to the E-stop loop effectively latches the equipment off, so the E-stop can be looped through other interlocks (IE, door sensors, safety curtains, etc) and any interruption of any of those will require a deliberate reset to resume operation. This also means that intermittent faults in any of those devices can be caught more easily.

author: Bootalito
author_rank: Unknown
timestamp: December 12, 2017, 11:35:39 pm
content: Quote from: ajb on December 12, 2017, 11:28:29 pmThanks. yeah I think you are right. If the estop loop opens or is intermittent it would still "latch" the circuit off requiring an deliberate reset. This helps me concrete the reasons why for this.

author: floobydust
author_rank: Super Contributor
timestamp: December 13, 2017, 05:02:52 am
content: If you're building electrical panels in the USA, you would design to UL508A and NFPA70, and have field inspection/certification at the very least.Unless you are an UL approved panel shop? Most customers reject an uncertified electrical panel, as it invalidates their site insurance policy.As far as fusing the 24VDC, I do it to be able to isolate devices/loads so the entire system doesn't collapse if one short occurs.Inspectors failed one panel a guy designed, he used ATO car fuses on the 24VDC rails. The inspector said it (fuse) must be rated to the power supply's input voltage (120VAC) as a primary-secondary short could occur and a car fuse is not rated to interrupt such a fault.

author: calexanian
author_rank: Super Contributor
timestamp: December 13, 2017, 06:58:01 pm
content: +1 for UL 508 requirements for most industrial applications. If you are doing this a lot, it is not hard to get you and your place of assembly credentialed. Both UL and ETL have options. As I recall its a class, then an at your facility training/inspection followed by occasional visits to make sure you are keeping proper records. Its about 5K to start as I recall and then yearly maintenance fees. nowhere near as severe as getting a product listed (As I am doing right now) As long as you have proper wiring diagrams, use all listed parts such as boxes, power supplies, relays etc, and follow proper wiring schemes its not hard. We do it with a local shop and I am looking at bringing it in house for when we do custom stuff.As for your 24 volt (What I am assuming is DC) bonding the negative side of the supply to earth (Not Neutral) is not uncommon. It reduces noise, etc, but two things. You cannot use the grounded conductor Again not neutral) as a conductor for that supply. A dedicated 24 volt pair of conductors will be required. Also people will have to be mindful of the polarity. Additionally you will need to check your local code on the mixing of low voltage and high voltage circuits. Its typically a No No. Low voltage typically requires a separate conduit, cable tray, etc. The earth ground bonding will be the same either way though. If you have any GFI breakers you need to completely separate everything.Just my initial thoughts.

author: duak
author_rank: Super Contributor
timestamp: December 13, 2017, 07:25:45 pm
content: About question 2, fusing a power supply output. Good engineering practice, as confirmed by electrical codes, indicate that the fuse is there to protect the weakest link which is usually the wiring. For example, with a 24 V, 2 A supply an output fuse wouldn't be used unless the wire to the distribution block was smaller than say 22 AWG. On the other hand, if the supply also powered some motor drive and was capable of 40 A, either an 8 AWG wire is used or a fuse limits the current to a level a smaller wire can safely handle.Cheers,

author: FoulkesBrau
author_rank: Newbie
timestamp: December 21, 2017, 09:43:39 am
content: Quote from: Bootalito on December 12, 2017, 07:05:13 pmIt does assist with fault finding, but there are other reasons behind this. I am going to take it you are talking about some sort of I/O panel here. In most cases your I/O cards would be on some sort of back plane which would have it's own power supply. You would have a second power supply (load power supply) which would then drive some other devices which need the extra power. Now depending on the "class" of power supply you have, the secondary side may or may not be tied down to the primary earth. You may have multiple analog signals that you are measuring in the form of 4-20mA etc. Those signals would be run in a twisted pair cable which in turn would have an external screen. The screen would be open ended on one side (usually the transmitter) and tied to a drain (earth) on the measurement side. This is to get rid of all the noise the signal may pick up from the field installation. Now lets go back to the two power supplies. Lets say that the two supplies are not at the same potential. This would interfere with how your card measures the signal (depending on the card type). It gets worse when you have a mixture of source and sync on analogue. In order to ensure that we are always at the same potential - we tie the secondary (0VDC) to the same reference - in this case earth. That way - no ground loops and no problems for the maintenance personnel.Now this is said assuming that you are not working in a hazardous (explosive) type environment. In the good old bad days, we used to deal with the hazardous environment issue by using a thing called a Zener barrier. The purpose of this device was to limit the energy transferred into the hazardous area for the signal measurement to such an extent that you would not be able to ignite any potentially explosive gasses around even if you were to short the two signal wires out and make them spark. The issue with this design, was that the barrier needed a place to dump this extra energy to. This is what is known as an intrinsic safe earth. Now this is a really big pain in the but to maintain and is just a headache. As you can imagine, the impedance of this earth is extremely low. To overcome this, some bright sparks invented the isolator. It works the same, except the safe side and hazardous side are galvanically isolated. But again, everything is at the same zero volt reference and we don't require the intrinsic safe earth.Somehow, this was never communicated to the younger guys and they just continued to design in the same manner without understanding why they were putting things in. You will see this often in instrument loop drawings where they might mention a thing called HQE (High quality earth) where they will tie the cable screen to and then hopefully tie that back to the main earth. So HQE and main earth are the same thing. You can find quite a lot on this if you look at the ISA website.Quote from: Bootalito on December 12, 2017, 07:05:13 pmWhy would you need to have a main fuse over the bank? The circuit breaker you refer to is only there to isolate the power - it is not there for protection. The fuses are the protection. Again, go look at many of the circuit breakers installed in your panels and then look at the tripping curve of the breaker - it will never protect anything in the panel, it is way too slow.Quote from: Bootalito on December 12, 2017, 07:05:13 pmIt is hard to say why you have a reset over a latched e-stop. Again, you need to look at the process control philosophy. Perhaps you would not like some machinery to start up automatically once you release the e-stop, perhaps you would like to do an inspection first before you return the circuit to service? It is hard to say without knowing the philosophy or the environment. It may even be a SIL requirement for your specific plant?Cheers