Contact Resitance

[templdl]

It seems simple to me to measure the voltage across the contact which represents the voltage drop as a result of contact resistance. The more difficult part would be to get a current reading at the same time. One couldn't get a more accurate value other than in actual operation conditions which may change everytime the contact is opened and closed which most likely would be trivial at best.
With these two figures you can calculate a real contact resistance. If you want to record the ambient temperature, humidity, the type of contact material and all that mundane stuff that doesn't really make any big difference include them in the documentation which will probably smoke those that are asking for the contact resistance but really are of no consequence. I have had those ask for measurements and documentation when they didn't have the slightest clue about what they were asking for.
In reading these replies I must say it is interesting how something so simple can be turned into such a big ordeal.

 

[tld11]

mityeltu:

In an attempt to answer the question I think you're asking I'm inferring the following details from your post regarding the situation you're in:

You work at a nuclear plant. The plant has an upcoming NRC Inspection (a Component Design Basis Inspection). As part of preparation for that inspection the site had a self-assessment performed. A member of that self-assessment team reviewed a sample of the site Safety Related Electrical Calcuations. In the plant DC Analyses that covers the safety related DC systems the reviewer identified a potential concern with the methodology. Namely that an explicit assumption was made that contact resistances were so small they were not considered in the voltage drop/voltage adequacy portion of the calculation, and no basis for that assumption was provided. The reviewer felt that this could constitute an 'unverified' assumption, which would not meet the site expectations for safety related caculations.

So right now no one is saying the results of the analysis are not valid, or that the voltage drop of any particular safety related circuit is excessive, or that Operability of any of the associated equipment is at risk, or even that the assumption isn't appropriate to make. The only issue right now is that the basis for the assumption isn't suitably documented in the calculation.

If the above is the case, then I would offer the following (note that if the validity of the calcs is in question and system Operability is a concern I would answer your post differently):

The general issue of 'inadequate' bases for assumptions in old calculations is not uncommon. In comes up frequently as plants tighten up their calculation processes and older calculations get revised for one reason or another.

I've seen this particular issue handled several ways in the dc analyses at different plants. Some where the contact resistances are not included or discussed at all, some where the contact resistances are not included and an explicit assumption is made similar to what you have, and some where a nominal resistance value (something like 1ohm) is added in the voltage drop calculation at the point of the load for every circuit to account for misc. like contact pair and wire termination resistances.

If I were in your situation I would go on to the INPO website (all operating plants in the US are INPO members), there is a spreadsheet there (at least as of a year ago) that consists of a pretty complete listing of all the System Engineers at the various nuclear plants. I'd start calling DC System Engineers until I came across one whose analyses included a similar assumption to the one used at your plant and ask them to send you a copy of what they used as a basis...usually people are pretty willing to help. It also might help to close the issue if you can come back and say that the assumption you're making is consistent with what others are doing.

If I couldn't find anyone with a sufficient basis to copy. Assuming you don't want to redo the calcs adding in some nominal resistance value, and assuming the rest of the calculation methodology is appropriately conservative, I would probably attempt to address it by writing a few generally worded paragraphs along the lines of the following, and include it the calc assumptions basis section:
- Note that contact pairs/wire termination hardware/etc. are designed and manufactured for the purpose of providing low impedance electrical connections.
- That the contact/connection resistances would be on the order of microohms to tens of milliohms.
- That the total circuit resistance of any circuit where voltage drop would be a concern would be on the order of ohms, which is an order of magnitude (or more) higher than the contact resistances. That large difference would be the basis for excluding the contact resistances.
- Unless there was a specific concern on an individual piece of equipment or circuit I'd shy away from measuring/documenting any specific resistance values. Contact resistances likely aren't measured during normal PMs (other than for circuit breakers) and the readings could potentially change quite a bit from measurement to measurement based on the amount of current flow, how recently switches/breakers were exercised, etc. Also, unless you're in an outage with the core offloaded it'd likely be difficult to do any kind of semi-invasive testing of a safety related circuit.

Some additional things to keep in mind when presenting this to whoever has to approve your resolution:
- The calculation methodology in whole has to be conservative. Every individual assumption/input has to be at least reasonable, but every individual assumption and input does not necessarily have to be absolutely conservative so long as the methology in whole is.
- An implicit assumption is made in almost every calculation (more or less) that safety related equipment is operating as designed. If Safety Related equipment is degraded (such as uncommonly high contact resistances) that is outside the scope of the calculation and would be an issue with the site implementation of 10CFR50.56 (Maintenance Rule).

 

[tld11]

I left out one important thing:

mityeltu:
If I couldn't find anyone with a sufficient basis to copy. Assuming you don't want to redo the calcs adding in some nominal resistance value, and assuming the rest of the calculation methodology is appropriately conservative, I would probably attempt to address it by writing a few generally worded paragraphs along the lines of the following, and include it the calc assumptions basis section:
- Note that contact pairs/wire termination hardware/etc. are designed and manufactured for the purpose of providing low impedance electrical connections.
- That the contact/connection resistances would be on the order of microohms to tens of milliohms.
- That the total circuit resistance of any circuit where voltage drop would be a concern would be on the order of ohms, which is an order of magnitude (or more) higher than the contact resistances. That large difference would be the basis for excluding the contact resistances.
QUOTE]

In addition to the above in the assumption basis I would note that all the equipment in the circuit path (contact pairs/wire termination hardware/etc.) is safety related equipment and is maintained in accordance with Tech Spec and Maintenance Rule requirements.

 

[weressl]

During my plant's last pre-CDBI (Component Design Bases Inspection) self assessment (this was performed before I started working here) an issue was discovered in a few of our calcs.

Apparently, it was decided that a statement that contact resistance of hand switches, breakers, limit switches, etc. being negligible was not an acceptable statement without some sort of supporting documentation. The basis for the assumption was that the voltage drop across these contacts would be insignificant when viewed against the voltage drop along the cable connected to these contacts.

These contacts are mostly associated with our 125V vital battery system, so I understand that after a number of hours when the voltage has decreased to near end of life of the batteries, the VD will be higher across the contacts, but this will still be very small in comparison to the VD along the cable.

My question is, how do I justify this statement if my justification as stated above is not acceptable? I have contacted some of the vendors for the SSC's but they are unwilling or unable to supply the information on the contact resistance.

Any thoughts on how to answer this?

In my previous life, in Europe, we did such calculations. It usually involved low voltage control systems with many series connected switches, such as construction elevators where one would add 'floors' as the building went up. Contact resistance deterioration was taken into consideration due to service lenght and potential mositure, corrosion. The data was based on historical information and a variety of components, such as push-buttons, limit switches and relay contacts. It is a complex issue and the general data is not necessarily available for ordinary mortals. (We had tables of data that were for internal use only.)

I am curious though the voltage deterioration you seem to take into consideration. I would rather calculate to the maximum safe discharge voltage, to which the battery bank should be sized for.