When to apply Protective Grounds on MV equipment?

[Shahzad]

Hello everyone,

I have a question mainly related to maintenance practice.

- Do we know when should we apply protective grounds while performing work on medium voltage equipment?
- If I have a 13.8kV breaker feeding a rectifier transformer. If people want to work on rectifier, then can we just rack out the breaker and consider it safe even if breaker does not have built in grounds? what is Code requirement ?
- Do I need to apply protective grounds at transformer junction box where 13.8kV outgoing cables are connected? I have seen some sites do that but some don't, so I am trying to find a reference as standard to refer?

 

[paulengr]

You apply protective grounds whenever hazardous voltages MAY be present. So consider the sources.

Examples of common sources are shielded cable capacitance, capacitors, inductive pickup, and lightning or wind effects. Another problem is if there is any chance of backfeeding from uncontrolled sources or if you cannot physically isolate everything . For example if homeowners could turn on a generator with the main closed or if your practice is tag only, not locked.

If all you have is unshielded cable without any sources present or that could appear, no reason to ground it. You are supposed to use a resistor but for instance I may have been known to just bridge a large screwdriver across the lugs in a starter.

Sometimes the sources cannot re-energize (capacitances) so simply draining is sufficient. Standard voltage detectors do NOT detect DC (a “tic” meter) so temporary grounding can be just that. Be aware of cable depolarization. Even if you short it out to 0 V after removing a ground it can easily recharge again as the dielectric molecules relax especially after DC Hi potting. The rule is wait 3 times longer than the test. So if you tested for 10 minutes wait at least 30 minutes.

 

[Shahzad]

You apply protective grounds whenever hazardous voltages MAY be present. So consider the sources.

Examples of common sources are shielded cable capacitance, capacitors, inductive pickup, and lightning or wind effects. Another problem is if there is any chance of backfeeding from uncontrolled sources or if you cannot physically isolate everything . For example if homeowners could turn on a generator with the main closed or if your practice is tag only, not locked.

If all you have is unshielded cable without any sources present or that could appear, no reason to ground it. You are supposed to use a resistor but for instance I may have been known to just bridge a large screwdriver across the lugs in a starter.

Sometimes the sources cannot re-energize (capacitances) so simply draining is sufficient. Standard voltage detectors do NOT detect DC (a “tic” meter) so temporary grounding can be just that. Be aware of cable depolarization. Even if you short it out to 0 V after removing a ground it can easily recharge again as the dielectric molecules relax especially after DC Hi potting. The rule is wait 3 times longer than the test. So if you tested for 10 minutes wait at least 30 minutes.

In my case, I have a highpower transformer which feeds a rectifier. Transformer incoming voltage is 13.8kV. When someone is required to work on cell lines which are connected directly to rectifier, we normally do lockout of 13.8kV breaker by racking it out and putting a lock. one of our site opens up transformer junction box and applies protective grounds on the cable connected. I don't understand why you need to do it.

 

[paulengr]

In my case, I have a highpower transformer which feeds a rectifier. Transformer incoming voltage is 13.8kV. When someone is required to work on cell lines which are connected directly to rectifier, we normally do lockout of 13.8kV breaker by racking it out and putting a lock. one of our site opens up transformer junction box and applies protective grounds on the cable connected. I don't understand why you need to do it.

It depends on how much shielded cable you have between the breaker and transformer.

It is standard practice with utilities because in almost every case they need it either because of actual or potential conditions. They don’t try to differentiate and this practice often carries over to operations who think they need to do it just because the utilities do it. Most linemen would think you are crazy for suggesting that grounding isn’t necessary but they don’t even know why they do it, just that it’s for “safety”.

Lots of industrial plants don’t do it, even when it probably should be done. And some have fairly home made “grounding trucks” that are one of the most dangerous devices ever invented. A grounding truck is a breaker chassis with the breaker removed and bus bars substituted so you can just rack it in for grounding.

My practice is I have a ground cluster and I know what it’s for and how to use it. I often see surge caps on MV motors and sometimes PF caps plus lots of MV cable. I sometimes use a hot stick with a ground to short cables or caps but more often than not I may use a screw driver or just let it discharge naturally. It really is job dependent.

 

[topgone]

It depends on how much shielded cable you have between the breaker and transformer.

It is standard practice with utilities because in almost every case they need it either because of actual or potential conditions. They don’t try to differentiate and this practice often carries over to operations who think they need to do it just because the utilities do it. Most linemen would think you are crazy for suggesting that grounding isn’t necessary but they don’t even know why they do it, just that it’s for “safety”.

Lots of industrial plants don’t do it, even when it probably should be done. And some have fairly home made “grounding trucks” that are one of the most dangerous devices ever invented. A grounding truck is a breaker chassis with the breaker removed and bus bars substituted so you can just rack it in for grounding.

My practice is I have a ground cluster and I know what it’s for and how to use it. I often see surge caps on MV motors and sometimes PF caps plus lots of MV cable. I sometimes use a hot stick with a ground to short cables or caps but more often than not I may use a screw driver or just let it discharge naturally. It really is job dependent.

I hate to pick on people but I don't feel comfy reading something like "just for safety"! We do LOTO because we need to cover our butts. A safe condition could be violated by persons or developments that are beyond one's control, that's why. I have seen people die because they missed to consider certain things and "ASSUMED" those things won't happen! One died because of a backfeed, another got scorched when lightning stroke hit a tower very far from his workstation, etc. I used to question what old people did in the past. Please don't learn about it the hard way!

 

[paulengr]

I hate to pick on people but I don't feel comfy reading something like "just for safety"! We do LOTO because we need to cover our butts. A safe condition could be violated by persons or developments that are beyond one's control, that's why. I have seen people die because they missed to consider certain things and "ASSUMED" those things won't happen! One died because of a backfeed, another got scorched when lightning stroke hit a tower very far from his workstation, etc. I used to question what old people did in the past. Please don't learn about it the hard way!

I thought I made it pretty clear. In a transmission line environment remote storms, wind, induction from nearby lines, and backfeeding are all very real concerns. It doesn’t apply to every case but happens so often it’s not worth looking for exceptions. In contrast in non-transmission line conditions it is more of an exception than the rule. It is very rare to need protective grounding in industrial plants. Most have no capacitors, short shielded cable, close parallel runs, they may have grounding switches or trucks, and distribution is purely radial. So all the conditions requiring protective grounding don’t exist.

Linemen feel perfectly comfortable tagging out equipment only or even calling the control room for a virtual clearance with remotely operated breakers. All perfectly legal and accepted practice in their situations but freaks out industrial plants who swear by physical locks. Utilities are abhorred by the idea of locking out breakers because there is no visible air gap and not using protective grounding. And don’t get me started on the whole thing of using gloves and hot sticks or whether sleeves are required when you’re not reaching over another energized line. Or whether gloves are necessary for testing for voltages on terminal blocks with guarded terminals (no exposed conductors).

All of these are examples of differences in work practices that vary from one operation to another. Depending on the operation they can be 100% legitimate and safe practices and often fall out if the differences between OSHA 1926, 1919.269, and 1910 Subchapter S. Each one works under different rules (construction, power generation/distribution, utilization), and so naturally we expect differences.

Taking the most extreme example many utilities have been making sleeves required on all jobs. And it is quite infamous that Eastern US linemen use gloves with hot sticks while nobody else does. In both cases it’s ridiculous. So yes I can say there are tons of work practices done “just for safety”.

My first point is that when it comes to situations like those described by OP protective grounding often is “just for safety” and has no justification for it whatsoever. Requiring protective grounds on a system that has no stored energy potential and no possible way to induce a potential is doing it “just for safety” and for no legitimate reason.