switchgear heaters 5

[tulum]

Hello,

When preparing a spec for switchgear when and where do you require stripheaters; and how do you size them?

Regards,
TULUM

 

[waross]

You require internal heaters when climatic changes may cause condensation in the switchgear. The typical condensation scenario is a cool night with high relative humidity. In the morning the air heats up faster than the internal metal parts of the switchgear. If the dew point of the ambient air rises above the temperature of the internal metal parts condensation will form on the cool parts.
Energized, loaded, switchgear wil typically keep itself warm enough to avoid problems.
If you have switchgear that is de-energized or unloaded at night in an unheated area then consider strip heaters. They just have to keep the internal parts a few degrees above the dew point.
Usually mounted on stand-off brackes fairly low so as to cause convection ait currents and spread the heat around.
yours

 

[dpc]

I would let the manufacturer size them. You will have given them environmental conditions and temperature range as part of basic spec. Anything outside or in an unheated environment would be a candidate. Some people buy them in all cases, then decide down the road if they want to use them or not.

We used to require the use of strip heaters rated at 240V but applied and sized to operate at 120V just to reduce odds of burnout (or burn up!).

 

[tulum]

Thanks guys.

Any ideas on what type of equipment needs them? IE I never see them in drives or starters (600V), but they seem to come standard (for us anyways) in switchgear (5kv and up)?

Is it just to protect insulators from tracking or all types equipment?

Rgards,
TULUM

 

[dpc]

Moisture can cause a lot of problems for relay contacts, contactors, breaker mechanisms, terminations, etc.

The breaker might sit in the closed position for 10 years straight, so you want to make sure it doesn't end up stuck there when you need to trip it.

 

[waross]

We use them on the alternators on the diesel generators to keep the windings dry when off line. The main alternators, not the battery charging alternators. The switchgear is energised 24/7 and stays dry from normal self heating.
yours

 

[stevenal]

Heat needs to be combined with venting to be effective. No point in setting up a convection cycle within the switchgear where the same moisture evaporates and re-condenses endlessly. Provide a vent at a high point for moisture laden air to escape, and another one low to provide the makeup air. The low one can also serve as a drain. If the switchgear is outdoors, assume moisture will always find its way in. The trick is to provide a way out. In my experience, manufacturers do not understand these concepts, and specifications should be clear in this matter.

One manufacturer provided a thermostat in the same high voltage cabinet with the heater and left it in the off position. Nothing in the instruction manual mentioned the need to even open this compartment prior to energization. The problem was discovered some time later when the breaker doors were blown open. Our spec now calls for thermostats to be located in the low voltage compartment.

 

[jraef]

tulum,
Your observation on drives and starters is acurate, but not because they don't need them, in fact the opposite is true. Anything with capacitors needs to be protected from freezing at least, and anything with PC boards is subject to tracking and shorting on the boards from moisture. It's unfortunate that not enough people take this seriously, but it's a fact nonetheless. People look at these expensive enclosure heaters and decide it isn't worth it, when in reality it can be done very inexpensively. I always use a simple little thermostat set to 40 deg. F feeding a strip heater stood off the inner panel with a few washers. That has never failed me on soft starters, drives and PLCs in outdoor and moisture laden areas. I know others who put in light bulbs as a heat source, although I don't recommend it because of how fragile the bulb is, but it does work in a pinch.




Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[stanlsimon]

Also be wary of moisture migration through conduits. This can be a problem in Agricultural facilities. The gear may be in a dry cool environment but vapor pressure will push water vapor from a warm wet region to the cooler region where moisture will condense or freeze, possibly blocking the trip action of the breakers.

 

[tulum]

Thanks again.

So if I should use them... any ideas on how to properly size them? The strip heaters we use on the 5kV-15kV gear are pretty simple caloritec ss type strip heaters. The heaters are sized to 300W no matter what size of enclosure... a little excessive I would think (100W-150W seems more resonable depending?).

I guess the question is... if I know the compartment size is there a simple calculation to determine the size?

Regards,
TULUM

 

[BJC]

One thing I have seen is swithgear heaters powered from a panel that is downstream from the switchgear. Fairly useless. You need the heat when the gear is de-energized. If you have two "trains" or "divisions" in your power system, power the heaters for one lineup of switchgear from the other. The same philsophy goes for motors.
Equipment that is energized and then turned off sucks up water as it cools.

 

[waross]

Good point BJC;
Another point, I'm not a big fan of thermostats. The cause of condensation in apparatus is often the air temperature rising and the relative humidity and the dew point rising with it, until the dew point is above the temperature of the slower heating metal mass. Our Generators are in an ambient temperature that much of the year ranges from the high 70s to the low or mid 90s. Condensation is more of a threat as the temperature rises than when it is falling. When the temperature is falling, the temperature of the metal mass is typically above the ambient temperature and so above the dew point. Condensation ocurrs at or below the dew point.
Put some warm metal in a cool moist environment and it doesn't attract condensation. Put a piece of cool metal in a warm moist environment and it immediately attracts condensation. Start to heat a thick piece of steel with a torch and it will sometimes condense moisture out of the flame. You have cool metal and very hot and moderately moist flame. Once the metal is heated above the effective dew point of the flame the condensation on the surface of the metal ceases.
You are not trying to dry the air, you are using the air to heat, via convection, the mass of metal. It doesn't have to be hot, just above the expected dew point.
Lose the thermostats, size the heaters as tulum suggests, and if you wish turn them off with a relay when the equipment is energised and producing enough internal heat to protect itself. Sizing; for a generator the heaters that I see are sized about 1 to 3 watts per KVA. 50 watts for a 50 KVA set does the job. When you get up to 3 watts per KVA I believe that it's a case of the nearest readily available size rather than a need for that much heat.
respectfully

 

[cuky2000]

For additional information on switchgear heaters and humidity control see the enclose sites.

http://www.powellind.com/main/Uploa... Switchgear and Anti-Condensation Heaters.pdf

http://www.heatersplus.com/enclosure.htm

http://www.b-line.com/product/ElectricalEnclosures/Thermal/heatcalchome.asp

 

[jghrist]

If you don't want to lose the thermostats as waross suggests, set them at 90 °F so they'll be on all the time except when the equipment heats things up more than that.

 

[mc5w]

Thermostats that have a remote bulbs inside of the switchgear and the contacts in the low voltage compartment would work well. Just make sure that you use a cross ambient ( oversize ) bulb. Both Allen-Bradley and SquareD make these and most other industrial control manufacturers make them too. You would need to use a dedicated conduit or opening for the capillary tube and ( obviously ) the conduit or opening needs to be big enoung to pass the bulb. Better yet, look up several manufacturers and spec the opening or conduit for the largest bulb that you can find so as not to be locked into a particular make and model.

Jraef is right about keeping drives above freezing. Electrolytic capacitors have water in them!

Starting batteries may also need a heater. Below about 40 or 45 dergrees Fahrenheit the electrical resistance of water based electrolyes increases dramatically partly because water molecules start clumping together which is how the idea of liquid crystals got started.

If you are in an arctic climate sulfur hexafluoride swutchgear also needs heaters to keep up the SF6 vapor pressure.

If ammonia gas or other corrosive gas is present then you would need to purge and pressurize the switchgear from say a regenerative blower that is across the street or other source of uncontaminated air. In this case you would need to dehumidify the air so that you are not introducing MORE moisture.

 

[tulum]

Thanks for all the help folks.

One more quick one. I am in the process of installing a type ss strip heater in a exisiting 5kV contactor compartment. I am told that all strip heater wires need to be run in a separate emt (or equivalent) - it is metal enclosed so there is 5kv at the back of the compartment...

Is this neccesary? There are control wires in the vicinity of where I am going to run the heater wires and they are just tie wrapped to the enclosure - no emt?

I am in Canada..

Regards,
TULUM

 

[stevenal]

Tulum,

Since heat rises the optimal location for the heater is at the bottom of the compartment. There should be minimal penetration into the high voltage compartment. I don't see what the conduit would buy you. Make sure you use heater wire. Regular wire tends to get crispy close to the heater.

 

[dpc]

At least in the US, any low voltage wiring in the high voltage termination section, must be isolated by a grounded metal barrier of some type.

 

[stevenal]

dpc,
Can you reference a standard? I believe most of our switchgear comes without any barrier. And wires must exit the conduit before connecting to heater terminals.

 

[tulum]

One more question on the strip heater circuit...

If I want to put a local/remote switch for the space heater power for (a) local when energized and (b) remote when deenergized is it advisable to break both the line and the neutral?

Typically, you would not break a neutral and the line in a control circuit, however, on this piece of mobile equipment, the remote will be a hubbel 120vac plug and the local is a on-board lighting panel.

I am not sure if I should just break the lines and let the remote neutral and local neutral be common, or break both and keep both neutrals separate? I would run a ground too...

seems trivial...but my pee size brain does not see an advantage to either...

Regards,
TULUM