Hi all,
new to the forum, so please forgive me if this has already been asked

We are in the middle of adding a management system (basically a box that tells the network what status the machine is in) to around 40 machines. Each box is integral to the machine, in that it is bolted inside it or onto the main machine and there will be 240Vac connected to it. The main machine has an isolator that kills the power, but the managment system will still have 240Vac on it from an external supply.
My question is, is this legal? I would have thought that an isolator should switch off all the power including the management system, as it is part of the machine once its bolted to it.
thanks,
Phil

Hi Phil -

Welcome to the forum!

I don't know the answer to "is it legal" in your location.

In my plant in the US, I have several machines with multiple power sources. Usually, there's the main power disconnect (480V) for this machine and then some type of control power circuit (120V) to interlock a line of machines together through an E-Stop or Run Allowed circuit. I use yellow wire to distinguish those interlocking signals that could be coming from an outside source & put a sticker on the outside of the cabinet that warns that there are multiple power sources within.

What would happen if the management system doesn't report to central because the machine disconnect is turned off? Can you put a lockable circuit breaker inside the machine's cabinet next to the management system to isolate that portion?

The key is going to be finding out what codes are applicable in your location. In the US, I use NFPA 79 which allows me to have multiple disconnects as long as I meet certain conditions - warning stickers, wire color code, etc.

HTH!
Susan

Multiple lockout situations are normal and common. What would you do if you are dealing with a double-ended substation which gets it's power from two sources? How about a ring station that can have even more?

The more important issue to consider in this case is bus protection. Since you have tapped power off the main bus at two locations, both devices must be large enough to shut off power under load in a short circuit scenario from the feeder for both. In addition, the feeder wiring must be large enough to withstand a short circuit from either one. If not, then you may have code/protection issues due to undersized protection for the wiring and/or disconnects (a potential fire hazard issue). When you wire up something similar to what you describe, it often results in a seemingly silly situation of an enormous disconnect for a relatively tiny circuit.

All codes I'm aware of require you to make it possible to kill power for maintenance purposes but leave it an open ended problem as to how many gyrations it takes to do this.

In addition, the emergency stop codes usually specify that you have to kill power to ACTUATORS (moving components) but not everything. So my common practice is to wire them up to kill power to the outputs on actuators only. Pilot lights, PLC's, input sensors, screens, and such all stay live. In addition this is a major troubleshooting aid because as long as nobody does anything to the equipment you can more or less infer exactly what state the system is in since the machine has STOPPED but all the sensors and machine states are frozen exactly where they are at (machine state is cleared when the reset sequence is triggered).

Hi there, thanks for that, how do I find out what code of practice I should be using? I dont mean to sound ignorant, but I really have no idea where to find out this info and our Health and Safety guy has no electrical knowledge at all

Phil



Hmmm, so what you mean is if one supply shorts out then they both must trip out? Or do you mean if both short together? The two supplies are not on the same bus from what I gather. This install has been done by contractors under management supervision. We (the maintanence guys) have tried to point out that we dont think it's being done correctly, but we really have no idea what is correct

Phil

[quote name='daxtojeiro' date='May 18 2009, 10:09 AM' post='82337']
[quote name='ssommers' post='82276' date='May 15 2009, 04:35 PM']
Hi Phil -

Welcome to the forum!

The key is going to be finding out what codes are applicable in your location. In the US, I use NFPA 79 which allows me to have multiple disconnects as long as I meet certain conditions - warning stickers, wire color code, etc.

HTH!
Susan
[/quote]
Hi there, thanks for that, how do I find out what code of practice I should be using? I dont mean to sound ignorant, but I really have no idea where to find out this info and our Health and Safety guy has no electrical knowledge at all

Phil
[/quote]

I don't know European codes, except that there are way too many of them and that the EU excels at creating giant unmanageable bureacracies! I'm not 100% sure but I believe it is IEC 60204. In the U.S., NFPA 79 is voluntary but since it's a consensus safety standard, it helps when something happens and the lawyers get involved.

[quote name='paulengr' post='82293' date='May 15 2009, 11:59 PM']

The more important issue to consider in this case is bus protection. Since you have tapped power off the main bus at two locations, both devices must be large enough to shut off power under load in a short circuit scenario from the feeder for both. In addition, the feeder wiring must be large enough to withstand a short circuit from either one. If not, then you may have code/protection issues due to undersized protection for the wiring and/or disconnects (a potential fire hazard issue). When you wire up something similar to what you describe, it often results in a seemingly silly situation of an enormous disconnect for a relatively tiny circuit.

[/quote]

Hmmm, so what you mean is if one supply shorts out then they both must trip out? Or do you mean if both short together? The two supplies are not on the same bus from what I gather. This install has been done by contractors under management supervision. We (the maintanence guys) have tried to point out that we dont think it's being done correctly, but we really have no idea what is correct

Phil
[/quote]

No, I'm not saying that they must both trip or short. What I mean is that I'm assuming that both are fed independently from a mains somewhere. What you will commonly find is this. Imagine that you have an existing machine with say #8 power conductors feeding the circuit breaker. Then this new add-on hardware that you are talking about gets added in. Well, they want it to be powered separately so they simply run #14 wire up and jam them down inside the lugs on the feed side of the primary disconnect. This creates three obvious safety violations:

1. The #14 wires are not adequately protected. The proper way is to use #8 wires to tap off the main disconnect switch to do this and run a second disconnect. However, this requires a separate enclosure with properly rated terminals to do it. Which brings us to the second issue...
2. The terminal lugs are almost gauranteed NOT to be designed or rated for running two wires on the same terminal because very few are rated for it. And...
3. The small #14 wires are probably far smaller than the rated wire size that the lugs on the main breaker are designed to accommodate.

The short circuit interrupting rating of the smaller disconnect or circuit breaker must also be large enough to handle the potential short circuit current that it is being fed with. This means that it can't be one of those tiny control circuit breakers that are commonly used in panels...it will probably be almost as large as the primary disconnect. Usually since the listing agencies and codes have standardized on circuit breaker interrupting ratings, this means that both circuit breakers will have the same ratings (such as 25kA, 65kA, 100kA, etc.).

So all I was saying is to watch out for these common screw-ups when someone does something like this.

Arrgh, OK, now I understand, thanks for that!
I will try and get to the bottom of what code we should be using, but as you say, there are a lot in the EU and most dont actually state exactly what you should do, they hint towards an idea rather than telling you what should be done

Phil

We have many panels with voltages coming in from other sources. This is usually due to interlinks for control situations ( sequencing for other systems)
I have also noticed some new panels coming with panel lighting supplys from the incoming side of isolators. These all have covers on the term strips and some indication on them that they will still be live when isolator off.

Here in the U.S. some industries require an external "lighting disconnect" on large control panels. This is a small panel usually on top of the main panel with a disconnect switch, transformer and GFCI receptacle. It takes 480 volt power from the line side of the main disconnect switch and provides 120 volts to panel interior lighting and utility receptacles. Square D still lists these as a standard product. As I recall, the utility circuits are wired in yellow to identify them as foreign voltage.

Mike