Discharging Capacitor banks per NFPA 70E 2021 Draft 2 2

[Wfg42438]

Hello Everyone,

As some of you may know the latest draft of NFPA 70E 2021 has a new annex describing calculations for discharge times for capacitor banks.

The calculation process is pretty straightforward however the calculation method refers to a discharge resistance.

I have looked online at many manufacturer datasheets for capacitor banks and noticed they mention the expected discharge time but not the discharge resistance.

Out of curiosity, I am wondering if anyone out there may have some field experience with this topic.

When reading annex R from the latest NFPA 70E draft I get the impression the discharge resistance should be a known parameter.

I did find that Schneider electric has an FAQ on this item however based on their description I get the impression that the discharge resistance is built in per capacitor within the banks.

Their sample calculation shows an expected discharge resistance of 150 kohm connected in parallel to each capacitor.

Not sure if anyone can shed some light on how the discharge process actually occurs in the field.

Do most manufacturers typically provide a discharge time for capacitor banks?
Which then simply requires workers to de-energize the capacitor banks (after a risk assessment that is) and then wait for the indicated discharge time specified by the manufacturer?


Or

Are there cases where working personnel discharge the capacitors via some external discharge resistance?
If this is the case what's the process to determine the discharge resistance?



Link to the NFPA webpage with the latest draft of 70E 2021

https://submittals.nfpa.org/TerraViewWeb/ViewerPage.jsp?id=70E-2018.ditamap&pubStatus=SDR

Link for Schneider Electric Sample Discharge Resistance FAQ:

https://www.se.com/ww/en/faqs/FA358405/

Image of Annex R Discharge Time Calculations

https://files.engineering.com/getfi...6b-41ed-938c-e98f4b95e7b4&file=Discharge2.jpg

Sample Data Sheet from Eaton describing Discharge Time:

https://files.engineering.com/getfi...249-4a0a-8aee-a86bd02a4157&file=Discharge.jpg

 

[stevenal]

OSHA 1910.269 says 5 minutes.
Workers in the field won’t be consulting an NFPA annex and performzing calculations. If 1910.269 doesn’t apply, find the section that does.
The ones I’m familiar with have internal resistors. Simply deenergize to start timing.

 

[che12345]

Dear Mr. Wfg42438

FYI: In the IEC world. the IEC Standards for capacitors <1kV are IEC 60831-x and 60931-x.

Q1. " ... The calculation ... refers to a discharge resistance..."
A1. Both above IEC Standards state: "...the residual voltage shall be 75V or less within 3min."[/b
The (discharge resistance [in ohm])is NOT a fixed value, it is dependent on the (capacity in [farad]) of the bank.
Q2. "... many manufacturer datasheets ... mention the expected discharge time but not the discharge resistance...".
A2. The {discharge time [in s ])is NOT a fixed value, it is dependent on the (discharge resistance [in ohm]) and the (capacity in [farad] ) of the bank.
Q3. ".. annex R ... latest NFPA 70E draft I get ... the discharge resistance should be a known parameter..."
A3. The (discharge resistance) is calculated based on, see above "...the residual voltage shall be 75V or less within 3min." "
Q4. ",,, Schneider electric ... FAQ... the discharge resistance is built in per capacitor within the banks.
Q4. Yes, within the banks.
Q5. " ... sample calculation shows ... discharge resistance of 150 kohm connected in parallel to each capacitor ...".
A5. ( discharge resistance of 150 kohm) is calculated for certain bank/capacity size only, in compliance with the Standards. It is NOT "one size fix all" banks, of various capacities.
Q6. "... shed some light on how the discharge process actually occurs in the field...".
A6. (how the discharge process actually occurs in the field), see above Standards.
Q7. "..Do most manufacturers typically provide a discharge time for capacitor banks? "
A7. No. (discharge time) is NOT the criterion.
Q8. "... requires workers to de-energize the capacitor banks (after a risk assessment that is) and then wait for the indicated discharge time specified by the manufacturer?..."
A8. No. (discharge time) is NOT the criterion.
Q9. "... Are there cases where working personnel discharge the capacitors via some external discharge resistance? If this is the case what's the process to determine the discharge resistance?..."
A9. Yes. (discharge the capacitors via some external discharge resistance) to ensure that the residual voltage is zero/dead before working on the equipment.
A similar case is DO NOT touch the conductor immediately after any HV tests. Discharge and earth the conductors for a "long" time before making any contact!
Che Kuan Yau (Singapore)

 

[Wfg42438]

@che12345

Based on your feedback I fully agree the discharge resistance is not one size fits all.

However, if you look at the attached Eaton data sheet you see that the manufacturer does provide the discharge time for the capacitor banks they make. They just don't specify any specific discharge resistor values which now makes sense since this will vary depending on the overall capacitance of the bank purchased. They are just saying the capacitor bank will have a specific discharge time for all kVar sizes they offer.

So that's, where my confusion lies, is there really a need to determine a discharge resistance or is that only determined for equipment where the capacitor banks do not have internal discharge resistance is not known?

Also, you made a comment that the external discharge resistance (bleed resistor) is in ohms, are you saying the discharge resistance is in the ohm range instead of kohm range?

 

[waross]

Has anyone mentioned the working voltage in addition to the bank capacity in regards to the value of the discharge resistor?
The law in Canada:

26-222 Drainage of stored charge of capacitors
(1) Capacitors shall be provided with a means of draining the stored charge.
(2) The draining means shall be such that the residual voltage will be reduced to 50 V or less after the capacitor
is disconnected from the source of supply
(a) within 1 min in the case of capacitors rated at 750 V or less; and
(b) within 5 min in the case of capacitors rated at more than 750 V.
(3) The discharge circuit shall be
(a) permanently connected to the terminals of the capacitor bank; or
(b) provided with automatic means of connecting it on removal of voltage from the line.
(4) The discharge circuit shall not be switched or connected by manual means.
(5) Motors, transformers, or other electrical equipment capable of constituting a suitable discharge path,
connected directly to capacitors without the interposition of a switch or overcurrent device, constitute a
suitable discharge path.

It will be difficult to locate PF correction capacitors that do not comply with Rule 26-222, (2) (a) or (b).
That is a design consideration and the discharge resistors are integral to PF capacitors.
As the capacitor is discharging through the resistor, the voltage, and the resulting current is also dropping.
You do the math.
The voltage in question is not the supply voltage but the instantaneous voltage at the moment of disconnection which may be as much as √2 times the RMS voltage.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear Mr. Wfg42438

Q1. "... Eaton data sheet ... does provide the discharge time for the capacitor banks they make.
A1. The IEC Standards DO NOT stipulate that (discharge time) shall be labelled. However,the manufacturer is NOT " prohibited" from giving any additional (informative} data.
Q2. "... don't specify any specific discharge resistor values... saying the capacitor bank will have a specific discharge time for all kVar sizes they offer.
A2. (discharge resistor value including the wattage ) is [selected and fitted] by the manufacturer; based on the standard that the product is tested. As a user, you DO NOT have to take care of the ohmic and wattage values, selected by the manufacturer. Caution: do NOT remove/alter the ohmic or the wattage values, selected by the manufacturer!
Q3. "... really a need to determine a discharge resistance or is ... equipment where the capacitor banks do not have internal discharge resistance is not known? "
A3. (discharge resistance ) would be a "fixed " value in order to comply with the IEC Standards, [residual voltage 75V or less within 3min]. BTW: the (resistance value) may be [rounded down] to the standard resistor value that is available on the market. This would result to the residual voltage to be lower than 75V or shorter than 3mim; which are acceptable.
A3.1 As per IEC IEC 60831-x and 60931-x, the banks SHALL be fitted with (internal discharge resistor), including additional requirements etc...
Q4. "... the discharge resistance (bleed resistor) is in ohms, ... is in the ohm range instead of kohm range? "
A4. The (basic value) of resistance is in [ ohm ]. To comply with the Standards, the calculated value may be in ohm, kohm range. It doesn't matter. FYI: 1 kOhm = 1,000 Ohm.
Che Kuan Yau (Singapore)

 

[Wfg42438]

@che12345

Thank you for the additional feedback.

In regards to the ohmic value of the discharge resistor, I was just curious about the range of typical values.
In other words, if typically the values for this discharge resistor is in ohm, kilo ohm, Mega ohm, etc sorry for not clarifying. However, as you stated this varies based on the kVar rating which means there may not be a "Typical" range for the value of this resistance unless you constantly deal with banks of the same size.

Based on your comments I now understand that the discharge resistance is not typically something the end-user determines. Manufacturers design their capacitor banks with built-in discharge resistors in order to meet the standard requirements for discharge times.

One last question is it more common for manufacturers to have the discharge resistance per capacitor or as an external resistance apparatus that is activated when draining the capacitor voltage?

 

[waross]

PF capacitors have internal resistors.
If a panel is custom designed using non PF specific capacitors without internal resistors it is the designers responsibility to provide a means to discharge the resistors.
This requirement may be met in a number of ways, including permanent connection to motor or transformer windings so that the capacitors may discharge through the windings.
Under Canadian rules, a custom panel may include automatic connection of discharge devices when the supply is disconnected.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear Mr. Wfg42438

Q. " ... is it more common for manufacturers to have the discharge resistance per capacitor or as an external resistance apparatus that is activated when draining the capacitor voltage?
A1. Reference IEC 60831-x and 60931-x for detail.
Capacitors tested to above IEC Standards SHALL have the (discharge resistors permanently connected across the terminals and other requirements [e.g. no fuse , no disconnect protective devices] etc...
A2. There are numerous capacitors for other purposes/tested to other standards that is WITHOUT discharge resistor on the market.
A3. It is NOT usual to have [an external resistance apparatus that is activated when draining the capacitor voltage].
A4. Reference above IEC standards for detail. Briefly, capacitor which is connected permanently across the motor winding [without any fuse, breaking device (i.e. contactor, breaker etc)] may not be fitted with the {discharge resistor}. This is taken into consideration that in general, the {winding resistance} is in the region of not exceeding say 50 Ohm; which would comply with the residual voltage and time requirements.
Che Kuan Yau (Singapore)

 

[Wfg42438]

Thank you Everyone for the guidance and feedback.
I will take a look at the references mentioned to better educate myself on this topic.