Monday, June 19, 2006

DDC - VAV Electrical Controllers Can Produce Faulty Electrical Output

It is usually required that each electrical DDC controller be powered by its own 24 VAC power supply (transformer). All wiring connections to the controller should be via copper conductors unless the manufacturer specifically states that aluminum wire is acceptable.

The wiring connections must all be made in accordance with the NationalElectrical Code and/or the local code as specified. Do not run Level 2 bus wiring in the same conduits as line voltage wiring(30 VAC or above) or wiring that switches power to inductive loads, such as contactors, coils, motors, generators, etc.

Only use shielded cable to run bus wiring. If the runs will exceed 500 feet, use only twisted shielded pair type wire Belden #28 gauge, Beldfoil 8760 or the equivalent as specified by the manufacturer of the controllers. If the runs will be greater than 500 feet, they will usually require a properly designed and selected repeater.(For more details on this topic check out my books on listed as NRC Publications #79, 84 and 37)

To further minimize sensor error caused by field wiring, unless otherwise specified by the controller manufacturer, the total resistance of all passive sensor wiring should be less than 3 ohms. Shielded cable will not generally be required if the sensor wiring runs are less than 50 feet.

When you are dealing with runs of between 50 feet to 100 feet, it is recommended that you utilize Belden #22 gauge, Beldfoil 8761 or the equivalent. With runs up to 250 feet, consider utilizing Belden #18 gauge, Beldfoil 8760 or an equivalent. With runs of up to 500 feet, consider utilizing Belden #16 gauge or Beldfoil 8719.

Wherever a manufacturer is mentioned above, it is for example only. I do not recommend one manufacturer over another. All engineers, users, designers etc must do their own research concerning product manufacturers they wish to utilize.

Wednesday, June 07, 2006

Death Doing the Right Thing

It was a hot humid NYC August day. About 10 A.M. it started to rain and rain and rain. By 1 P.M. it was so dark it looked like midnight. Thunder and lightning occurred almost every 5 minutes and water kept coming and coming. By 2 A.M the next morning there was water everywhere.

Morgan Towers is a 12 story coop in Brooklyn Heights. At the time it was only three months old and was 80% occupied. Water was coming in to the basement area from everywhere. By about 4 A.M. the basement area was flooded and water was pouring down into the elevator pits. The electricity went out and the emergency generator came on. It stayed on for about 30 minutes and failed.

By 8 A.M the rain had stopped, the clouds were moving by fast and slight ribbons of sun light were streaming into the easterly windows.

By 12 P.M. electricians arrived to see what they could do to get the electricity on in the building. The water had receded and the buildings staff was in the process of cleaning up all the mud and dirt. The generator had tripped due to shorts in many of the circuits caused by the flooding.

The electricians were moving along checking circuits and repairing what they could. New wiring and circuit boards were required in many rooms. It was about 90 degrees outside now and about 110 in many of the basement rooms. They were now working in the main air conditioning equipment room. It was very hot, and late. It was now about 9 P.M. and the two electricians remaining in the building were very tired. They skipped meals and were existing on orange juice.

I investigated what happened next for 6 months, the following is a very short version of what I pieced together.

The master electrician went to the back of the A/C room while his helper stayed at the main panel to make sure no one turned any of the power on. They used no lock out tags and relied strictly on each other. It was now about 10 P.M., they were exhausted, but determined to get the building air conditioning on. The master electrician moved circuit by circuit checking for shorts, damaged wire and anything else he could find. He found and repaired many damaged circuits, making many temporary repairs. By this time the master electrician was now out of the line of sight from the helper standing guard by the panels.

From my investigation it appeared the electrician was resting on one of the pieces of equipment, drinking a bottle of juice. It must have been quiet, because about this time the helper yelled out for the electrician to see if he was O.K. We have no way of knowing what the electrician said next but the helper swore he heard something that sounded like "A.O.K. turn the power on" a phrase they had used many times before. The helper testified that almost at the instant he turned the power on he heard a loud scream. He shut down the power and ran to where he heard the scream; only to see his partner on the ground shaking. He immediately gave him CPR and called EMS on his cell. He lost much time as he fiddled with his phone shaking all over. The records showed EMS arrived in about 9 minutes. His partner died on the way to the hospital.

Here he was, a guy trying to do the right thing...and yet he died.

All to often, bad out comes are a result of many different small mistakes adding up to a major disaster. Try as we may, in attempting to get workers to pay attention to the smallest safety error, we find that they try and spend time watching out for the big mistake. They ignore the many small problems that can pile up and turn into a major disaster.

Allowing themselves to become exhausted set the stage; add in working with low blood sugar and the die was cast. They than proceeded with out utilizing lock outs and let themselves get out of each others direct line of sight. They were now heading down hill full speed. Add about 20 technical errors and you have the making of a major disaster, which of course it was.

On construction jobs major disasters are rare but accidents due to the compilation of many small things overlooked are very common. When we tear apart a major accident we find it has grown from ignoring many smaller signs that occurred from ignoring many smaller infractions. Pay attention to the micro event and you may avoid the major disaster.

Be safe,