Problem : Short of one wire in a cable, how to fix it?

From what u state I understand you are one core short in a multi core control cable? is it correct?

The idea of adding the YL/GN on the outside is fine, if your customer will accept it. I'd certainly try to get away with it! Yellow/green is generally reserved for Protective Earth, so they may get funny about it.

For single core YL/GN, try e.g. RS Components . You can click Switchgear Cable to go directly to the appropriate entries.

They stock 34 varieties, with CSA from 0.5mm² to 25mm²

What you really need to check is whether this machine must be approved by a notified body (UL, CSA, etc) and, if so, whether they will accept an unapproved cable. Many won't. It sounds for all the world like a power cable safety ground; if so, replace it.

I agree that the missing core is probably a Safety Ground (Protective Earth), but it depends on the wording of the relevant spec. as to whether or not the proposed solution will be accepted.

If the original cable, the added PE conductor and the new outer sheath, and the methods used, are each individually approved (or even not disapproved!), it may be possible to put forward a case for accepting it as an alternative. If these criteria are met, the customer may be persuaded. If not, no chance.

The bottom line is that someone has screwed up, and the customer is well within their rights to throw it out, & say "fix it now - or we want our money back" - and that's for the whole job!

If you (OP) are on good terms with your customer, they're desperate for the system to be up'n'running, and the Safety requirements can be met, you may get away with it.

I'd advise that, if the added core solution is accepted, you replace the cable as soon as possible (liaising with your customer to minimize disruption) with the correct cable. In this case the cost of the cable could be the difference between the survival and downfall of your company.

I also advise whoever was responsible to be a lot more careful next time.

You're on the right track, use original wire tape plus additional three wire group run along side in insulation respective and admit two wires dummies. will all pass inspections no worries.

If the customer is worth two wires of waste then call it experience...

Yellow/Green is generally used for earth. If the customer approves you may try earthing the machine at several loactions and finally connect a thick copper wire from the machine to the earth.

Confirm if this is approved in your region before you make this suggestion to your customer

Are you missing one wire or are you using a different color in place of the yel/green?

If you are missing the wire altogether the customer is right. Just buy the correct cable and send it to him. Most cable distributors can sell you the amount you need.

If you are using a different color in place of Yel/grn Then buy the correct wire and splice it at both ends if they agree.

Useable or not you need to meet the customer requirements, You screwed up and need to satisfy them. Your repeat orders may depend on this.