Policy for working on VSD/VFD while live.

[AlanGarbers]

Our company's policy is to not allow HVAC mechanics towork on live voltages over 600 volts, even with the proper PPE on.
However, many of the newer compressors are driven by VFDs that produce DCvoltages up to 750 volts. In the troubleshooting procedures we need to checktest points on the drive and are exposed to these voltages.
After bringing this up it was decided that the policy basically applies tosupply voltage, not what is generated by the drive.
I have tried searching for standard recommended practice from drive makers butI find nothing on the topic.
I would like to know what policies and procedures other companies use insituations like this.
Thanks.

 

[Besoeker]

After bringing this up it was decided that the policy basically applies tosupply voltage, not what is generated by the drive.

If the supply voltage is 600Vac its peak is about 850V. That's where yout DC link voltage comes from. It is no higher than the peak of your AC supply.
Personally, I wouldn't want to work on either when live.

 

[petersonra]

Our company's policy is to not allow HVAC mechanics towork on live voltages over 600 volts, even with the proper PPE on.
However, many of the newer compressors are driven by VFDs that produce DCvoltages up to 750 volts. In the troubleshooting procedures we need to checktest points on the drive and are exposed to these voltages.
After bringing this up it was decided that the policy basically applies tosupply voltage, not what is generated by the drive.
I have tried searching for standard recommended practice from drive makers butI find nothing on the topic.
I would like to know what policies and procedures other companies use insituations like this.
Thanks.

most drives have displays where the dc voltage can be read off the display so why would you need any exposure at all to the live voltage?

 

[Besoeker]

most drives have displays where the dc voltage can be read off the display so why would you need any exposure at all to the live voltage?

It's a point I pondered too.
If you want to make a measurement, connect the instrument with the drive dead then power it up.

 

[AlanGarbers]

The problem by powering the drivedown and then back up is that it often clears an intermittent fault. The troublingshooting procedure often requires taking various readings at multiple points onthe drive while the problem is occurring.
Thanks for your input. The first response is what I was looking for.

 

[Jraef]

The problem by powering the drivedown and then back up is that it often clears an intermittent fault. The troublingshooting procedure often requires taking various readings at multiple points onthe drive while the problem is occurring.
Thanks for your input. The first response is what I was looking for.

Right, I get that, I go though it all the time. But still, the point is that when we say that a voltage is "600V", that means that it is 600V RMS, but the real PEAK voltage of every sine wave is actually 600 x the sq. rt. of 2 (1.4142135), so 848V. Technically, this is "over 600V" too, but basically, we all should know what it means and accept it as the conventional "nominal" value, not a strict literal number. So the peak voltage potential inside of the DC bus of a VFD is not any worse than the peak voltage potential of the AC feeding it. There are inherent differences in the dangers of it being DC vs AC and whomever works on a live VFD should be trained and educated on those differences, as well has NEVER allowed to be working on it alone. That is a separate matter.

But all of that should be part of a basic Safe Electrical Work Practices program administered by the facility owners, for which EVERY person opening that enclosure would needs be certified on, all of which is REQUIRED by OSHA. So if the HVAC technicians are not set up and trained as Qualified Electrical Workers, certified every year under the facility owner's program, then they should not be opening that box regardless of what is inside. Also, hiring an outside contractor to do that does NOT let the owners and managers of the facility off the hook.

 

[WarAxe]

Our company's policy is to not allow HVAC mechanics towork on live voltages over 600 volts, even with the proper PPE on.
However, many of the newer compressors are driven by VFDs that produce DCvoltages up to 750 volts. In the troubleshooting procedures we need to checktest points on the drive and are exposed to these voltages.
After bringing this up it was decided that the policy basically applies tosupply voltage, not what is generated by the drive.
I have tried searching for standard recommended practice from drive makers butI find nothing on the topic.
I would like to know what policies and procedures other companies use insituations like this.
Thanks.

We have a policy that unless there's a compelling reason to have to keep it energized, it must be locked out. In your case, if it's locked out you cannot perform troubleshooting in accordance with the operating manual... so that'd be a good reason. Making the distinction between "supply" voltage and "output" voltage sounds really silly to me.

Can you install guarding with holes for test probes?

 

[pfalcon]

The problem by powering the drivedown and then back up is that it often clears an intermittent fault. The troublingshooting procedure often requires taking various readings at multiple points onthe drive while the problem is occurring.
Thanks for your input. The first response is what I was looking for.

Common problem for troubleshooting. Faults that disappear when you reboot, recycle, reuse, etc. VFDs are now sold with both remote or attached displays. Also displays can be permanent or temporary. If this fault is painful enough to justify live work then it's painful enough to justify using a VFD with a remote or mounted display permanently attached. The upcost pays for itself in the "time to fix" for the first failure.

 

[meternerd]

It's a point I pondered too.
If you want to make a measurement, connect the instrument with the drive dead then power it up.

Be nice if it was that simple. But the reality is that you cannot always troubleshoot a de-enrgized drive any more than you can figure out why the engine in your car is "making a weird noise" (like my daughter is fond of saying) without starting it first. Just take the time to minimize the risks and wear your PPE!

 

[Besoeker]

Be nice if it was that simple. But the reality is that you cannot always troubleshoot a de-enrgized drive!

That's not what I suggested.

 

[meternerd]

That's not what I suggested.

Sorry...no offense meant. It's just that as electricians, we always get the "Why can't you just tag it out first?" when we try to explain why we need to work on live equipment. We try to explain, ask them to accompany us to a jobsite, etc., but they always seem "too busy". Kinda frustrating sometimes. Bottom line is, you do what you have to do as safely as you can.

 

[Besoeker]

Sorry...no offense meant. It's just that as electricians, we always get the "Why can't you just tag it out first?" when we try to explain why we need to work on live equipment. We try to explain, ask them to accompany us to a jobsite, etc., but they always seem "too busy". Kinda frustrating sometimes. Bottom line is, you do what you have to do as safely as you can.

No offence taken. There are times when live testing is unavoidable. The topic was about variable speed drives. We sometimes do DC fed units where the DC can be upwards of 700V. To measure that I'd first connect a meter before powering up the DC bars.

As I've got older. if no wiser, I've become a bit more circumspect about risk and avoidance.

A salutary tale.
A DC fed drive in a production facility had blown a fuse.
As ever, production is king. Getting the equipment back in service in the shortest possible time puts pressure on people.
The guy who went to change the fuse did not heed the warning notices about removing all supplies. And didn't. He knocked off the auxiliary AC but not the main DC.
The outcome was serious second degree burns, a long spell in hospital and skin grafts.

I've been back a number of times since to do maintenance checks and every time I see that drive with the burned holes and, what look like burnt bits of skin embedded in the paintwork, it makes me think about risk. More particularly, what I need to do to avoid it.