Having this wiring standard:
Instrument -> (single pair wire) -> Junction-box -> (multi-cable) -> Marshaling cabinet -> (wires) -> DCS I/O

What is the best way to proceed with the spare wires from the junction-box to the marshaling cabinet? Shall we connect the spare wiring (inside the multicable) to the marshaling cabinet terminals or leave it exposed?

In case we do not have the marshaling cabinet, that is:
Instrument -> (single pair wire) -> Junction-box -> (multi-cable) -> DCS I/O

What is the best way to proceed in this case with the spare wiring inside the multi-cable once it gets to the DCS I/O terminals?

Thanks in advance.

 

I don't know if it's the best way, but we just leave them in the bottom of the panel with tags identifying them as spares and where they go.

As infrequently as we have needed spares, this method works just fine.

> What is the best way to proceed in this case with the spare wiring inside the multi-cable once it gets to the DCS I/O terminals?

 

So you leave the wires exposed (not connected to any terminal)? I assume you are talking about the second case (no marshaling cabinet).

What would you do if you have a marshaling cabinet? Would you connect the spares to the marshaling terminals in this case?

Thanks!

> I don't know if it's the best way, but we just leave them in the bottom of the panel with
> tags identifying them as spares and where they go.

> As infrequently as we have needed spares, this method works just fine.

> What is the best way to proceed in this case with the spare wiring inside
> the multi-cable once it gets to the DCS I/O terminals?

 

Well, I'm not sure what a "marshaling cabinet" is, I can only assume it's the equivalent of our main panel. You know what they say about a people separated by a common language...

But when we design a system, we add in plenty of spares. Spares that go to nowhere special, and are left unterminated at both ends.

After 25 years of this practice (25 years just at this site, for forever elsewhere) we haven't had any issues.

Our newer design wires the spares Rockwell DNB modules and power supplies for a DeviceNet system all the way to the DeviceNet connector mounted on the side of the panel.

But this is for convenience should we add another DeviceNet network to the system (I don't believe we ever have here in the Paint shop, so it amounts to a source of spare DNB modules and power supplies for us )

 

You absolutely should not leave them coiled up and unconnected.

The spare cores should be connected to spare terminals if you have the space and the other side of the terminals <b>MUST</b> be connected to earth. Check local regulations but its usually instrument, clean or intrinsically safe earth.

Preferably the connection to earth should be duplicated one at the top and one at the bottom of the set of spare terminals. You can daisy chain several sets of this in a rail so that only 2 earth wires come from each rail of field cable terminals and take them to earth.

The reason they should be connected to earth is to prevent any personnel hazard (and in hazardous areas explosion hazard) in the event of a fault in the field placing a voltage on the spare cores.

 

>You absolutely should not leave them coiled up and unconnected.

Well, we didn't start out this conversation talking about IS wiring. In our conveyor panels it's just peachy to leave them unconnected, and I doubt that we will be changing anything anytime soon.

But now that you brought up IS wiring, well yeah, duh. Something about stored electrical charge and all...

So what's your position on wiring that goes into an Intrinsically Safe area in explosion-proof conduit?

For things like Start/Stop stations we (obviously) wire these up in hard conduit with explosion-proof boxes, seal-offs, and whatever else required. It sure would be *nice* to have a spare or two down on the floor...

 

This is another one of those personal/site preference issues--very similar to where should the shield drain wire be terminated to earth (ground), and should it be continuous through multiple JBs or not. However, there are some local codes and regulations which do require one end of the cables to be terminated and earthed (grounded). And, similar to what color wires should be used for positive and negative, and line and neutral.

A marshaling cabinet is, by most definitions, an intermediate cabinet very close to the cabinet/panel the control system is housed in. Some people (very strongly) feel that field wires should NOT be terminated directly on control system terminal boards. So, usually at the factory, there are wires/cables which connect one side of the terminal boards in the marshaling cabinet to the control system terminal boards. And the field wires/cables are terminated on the other side of the terminals in the marshaling cabinet. It adds significant cost to the project if there are a large number of wires and cables, especially when a lot of twisted, shielded pairs and three-wire, twisted, shielded cables have to be used between the marshaling cabinet and the control system terminal boards.

It does sometimes look nicer (terminating field wires in a marshaling cabinet)--depending on the quality of the electricians and their supervision. And, it also depends on how well the marshaling cabinet was laid out.

The correct answer to this query is: Do what local codes and regulations require, first; then follow plant/site practices; then do what the installing supervision requires in the absence of any other guidance (hopefully, the installing supervisors have researched the local codes and regulations and asked the plant about their practices and requirements!).

I have primarily just left the unused conductors coiled neatly in the bottom of the cabinet--and take great care to ensure the electricians properly mark the cables so they can be easily identified in the future when looking for spare conductors for some addition to the control system. I have, when budget allowed, used crimp-on connectors which are like a "cap" on the ends of the conductors in the control panel. I always make sure the ends of unused conductors in local junction boxes in the field are either terminated (provided there are enough terminals provided), or individually taped and coiled/zip-tied neatly out of the way. And, I've never heard of anyone getting shocked or sparks--providing the proper separation was used between high voltage/high current conductors and signal level conductors.

Local codes and regulations should always take precedence, followed by site practices or requirements (which are sometimes stated clearly in the request for proposal/contract).

 

I'm sorry gents but I can't quite believe what I'm reading. Take care of yourselves and your workmates.

 

I think CSA sums this up, it depends on what regulations apply. For Zone 1 all spares must be terminated in a junction box on a terminal rail. I'm not so keen on just leaving them taped up in the bottom of the panel, people step in there etc and the cables get dirty and damaged. Most marshalling panels have some spare terminations, I would terminate them there and if you have knife switches, leave them open, at the machine end, try and find or install some spare terminal strip.

 

Just to clarify:

Installation with marshaling cabinet:
The spare wires will be connected to the marshaling terminals (field side) and then connected to ground.

Installation without marshaling cabinet (field wires connected to DCS I/O cards directly):

The spare wires will be left unconnected coiled in the bottom of the cabinet. Will be right to connect them to ground? How to do it properly?

Thanks!

> I think CSA sums this up, it depends on what regulations apply. For Zone 1 all spares must be terminated in a
> junction box on a terminal rail. I'm not so keen on just leaving them taped up in the bottom of the panel, people
> step in there etc and the cables get dirty and damaged. Most marshaling panels have some spare terminations, I would
> terminate them there and if you have knife switches, leave them open, at the machine end, try and find or install some
> spare terminal strip.