BMS Interview questions and answers

I have listed some basic interview questions and answers for building automation industry and I used to update this post frequently.

Based on the companies, Job requirements; technical interview questions will be varied.

If you are fresher and looking for a job in BMS industry in entry-level positions, then these questions will be helpful for you and they will try to assess you whether you can able to learn and work in BMS industry and not expect you as an expert in BMS related knowledge, instead, they will check whether you showing interest and ready to learn BMS.

If you are experienced in BMS already and looking for further career development, Then the questions will be based on the product knowledge already has, and other non-technical questions like salary package, communications, attitude, team player and other soft-skills.

In upcoming my next post, let me try to write detailed about the how to get the jobs in the various industry as entry-level and experienced level by understanding the various expectation of employers/interviewers especially in automation Engineer field. Keep in touch/Subscribe with this blog to get further updates.

In this post I have listed/limited/expected some basic technical BMS interview questions and answers(Google/Wikipedia it) for better understanding.

Checkout some important blog content before you go to interview for some ideas about BMS.

Read more : 

Understand the Basic concept of BMS system 

Building Management System course

Understand the BACnet Protocol

What is DDC or Direct Digital Controller in BMS System?

This following BMS interview questions and answer purely focused on testing and commissioning engineer, or related to field works positions.

For other roles like designing, Estimations, Project engineer; In addition to the below questions, the technical expectation will be different. 

What is the BMS System?

BMS otherwise called as BAS or building automation. It is a computer-based control system which reduces the manpower, automate the system, and saving the energy consumption in buildings by monitoring and controlling the mechanical and electrical equipment in modern-day buildings or any industrial plants.

What is the main component of the BMS System?

Hardware

• DDC-Direct digital controller
• Sensors
• Actuators
• Cables to connect sensors, actuators to DDC.
• HMI display-Human machine interface.
• PC Workstation
• Server to save the large database.

Software

• Programming or configuration tools.
• Graphics or User interface.

Networking protocols

• TCP/IP– Transfer control protocols/Internet Protocol.
• BACnet– Building automation controller network-ASHRAE
• Modbus
• LONworks
• CANbus
• and numerous protocols available.

What is the purpose of the BMS System?

• Monitoring and controlling the field equipment.
• To reduce manpower in the building.
• To provide comfort and healthcare for the building the tenants.
• To maintain the mechanical and electrical equipment in the buildings.
• Record all the events occurs in building machines and raise alarm if any malfunction happens.

What are the basic features of BMS Software?

• Log the events
• Alarms.
• Reporting
• Scheduling.
• Graphics.

Block diagram of BMS Architecture

Management Level: This is the front end for operator and engineer used to visualise the graphics for controlling and monitoring the systems which have computer workstation, server, web browser, printers.

Automation Level: BMS Router and other main controllers connected in building network integrate the third-party system and connect BMS devices.

Field devices Level: this is Level where BMS controllers connect to field systems sensors, actuators, and other panel circuits to monitor and control.

What is Trend log?

Trend log records and displays the behaviour of an input or output associated with BMS.

Trend logs help you troubleshoot problem areas and identify critical operating trends in your system. For example, log the temperature in a room to see if the system is keeping it within desired set points.

What is the Alarm?

Alarms monitor equipment or environments in a building and alert someone when conditions are outside the defined normal range.

What is Report?

We can generate a report for any type of alarm, Trend log or events in the required format for over a period of time

What is Scheduling?

Schedules are a type of object in BMS Controllers and it can be configured to start and stop the machines pre-defined time.

Schedules enable you to plan system operations based on time of day, special events, and holidays. The combination of a standard, holiday, and event schedule is called a schedule set.

What are the optimum Starts?

Optimum start adjusts the heating or cooling start time in an attempt to bring zone temperatures to the appropriate setpoint for occupancy. For example, you want the classrooms in your school to be at a comfortable temperature (their individual set points) before class begins. You can do this by first setting up each classroom as a zone and linking a schedule to each zone.

Then, you set up an optimum start for each zone (classroom). Optimum start calculates when to start warming or cooling the classrooms based on the settings you define.

What is Micro-controllers?

A microcontroller is an integrated circuit (IC) which is small, low cost and self-contained computer designed to handle a specific task in embedded systems. In simple words, a microcontroller (MCU or Microcontroller Unit) is a small computer integrated into a single chip.

What is Resistors?

A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses.

What is Ohms law?

Ohm’s law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship

What is Relay?

Relays are the switches which aim at closing and opening the circuits electronically as well as electromechanically. It controls the opening and closing of the circuit contacts of an electronic circuit. When the relay contact is open (NO), the relay isn’t energized with the open contact. However, if it is closed (NC), the relay isn’t energize given the closed contact. However, when energy (electricity or charge) is supplied, the states are prone to change

What is DDC Controllers?

DDC is a controller which use the analogue or digital signals from various devices of a field sensor and actuators and then process and control the system based on the program written inside the controllers and has the capability to sends the information to another controller or DDC.

Digital Input – DI

Digital inputs are binary inputs (0 or 1) that are applied to the DDC. Binary inputs are basically voltages, varying from 5V to 230V depending upon the type of card used. In simple words, any push-button, switches or sensors produces digital inputs to a DDC. Digital inputs are used to check the status of any devices whether it is ON or OFF. For an Input module of 24Vdc, 0Vdc acts as OFF state (Binary 0) and 24Vdc acts as ON state (Binary 1).

Digital Output – DO

Digital outputs are binary outputs (0 or 1) from the DDC. It is a processed control output from the DDC to the field. It is used to ON or OFF any piece of field equipment. DO is like a contact of a relay when the preprogrammed conditions are satisfied the contacts are closed. DO can be used to operate solenoid valves, relays, indicating lamps or as a command to any other devices.

Analog Input – AI

Analog input is a continuous input from the field to the DDC. Unlike digital signals, is not a constant voltage signal. It can vary depending on the field conditions. The commonly used analogue signals are 4-20mA signal and 0-10Vdc Signal. Depending on the field conditions the input signal can vary between 4-20mA or 0-10Vdc. For Example, if you are monitoring a pressure of water flowing through a pipe using a pressure transmitter of range 0-10 Bar, the transmitter will give a signal between 0 and 10V proportional to the pressure in the pipe.

Analog Output – AO

Analogue output is a continuous output from DDC to the field devices. For example, if you have a variable frequency drive and you want to give a speed reference signal to it you can use analogue output. Otherwise, if you are measuring the pressure using analogue input as explained above and you would like to display it on an analogue meter you can use an analogue output of DDC.

Read more on https://www.electricalclassroom.com/digital-i-o-and-analog-i-o/

What is meant by Frequency?

Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency. Frequency is measured in units of hertz (Hz) which is equal to one occurrence of a repeating event per second.

what is Pulse width modulation?

Pulse Width Modulation (PWM) uses digital signals to control power applications, as well as being fairly easy to convert back to analogue with a minimum of hardware.

One of the parameters of any square wave is duty cycle. Most square waves are 50%, this is the norm when discussing them, but they don’t have to be symmetrical. The ON time can be varied completely between the signal being off to being fully on, 0% to 100%, and all ranges between.

Shown below are examples of a 10%, 50%, and 90% duty cycle. While the frequency is the same for each, this is not a requirement.

Image Credit:https://www.allaboutcircuits.com/textbook/semiconductors/chpt-11/pulse-width-modulation/

What are Open-loop and closed-loop control system?

Open Loop Control System

In the open-loop control system, the output does not affect the control action of the system. In other words, the system whose working depends on time is known as the open-loop control system. The open-loop system is free from the feedback. Let’s understand this with the help of the few examples.

Example 1: Consider the clothes dryer whose control action is done manually by the operator. Depending on the wetness of the clothes suppose the operator set the timer for 30 minutes. After 30 minutes the timer will stop even after the clothes are wet.

The dryer stops working even if the desired output is not obtained. This shows that the system has no feedback. Here clothes dryer is the example of the open-loop system and the timer is the controller of the system.

Example 2: The automatic washing machine is an example of the open-loop system. The operator manually sets the operating time of the machine. The machine stops operating after the set time, even the desire cleanliness of clothes are not obtained. This happens because the machine has no feedback system which signals the control action of the system for the desired output.

The open-loop system is simple, require less maintenance. Also, it is fast in operation and very economical. But the accuracy of the system is less, and it is less reliable.

Closed-Loop Control System

The closed-loop control system means the output of the system depends on their input. The system has one or more feedback loops between its output and input. The closed-loop system design in such a way that they automatically provide the desired output by comparing it with the actual input. The closed-loop system generates the error signal which is the difference between the input and output.

Image/content Credit : https://circuitglobe.com/difference-between-open-loop-and-closed-loop-system.html

What is PID?

As the name suggests, this article is going to give a precise idea about the structure and working of the PID controller. However going into details, let us get an introduction about PID controllers.

PID controllers are found in a wide range of applications for industrial process control. Approximately 95% of the closed-loop operations of industrial automation sector use PID controllers. PID stands for Proportional-Integral-Derivative. These three controllers are combined in such a way that it produces a control signal

What is Database?

A database is an organized collection of data, generally stored and accessed electronically from a computer system. Where databases are more complex they are often developed using formal design and modeling techniques.

The database management system (DBMS) is the software that interacts with end-users, applications, and the database itself to capture and analyze the data. The DBMS software additionally encompasses the core facilities provided to administer the database. The total of the database, the DBMS and the associated applications can be referred to as a “database system”. Often the term “database” is also used to loosely refer to any of the DBMS, the database system or an application associated with the database.

What is SQL Database?

SQL is the standard language for dealing with Relational Databases. SQL can be used to insert, search, update, and delete database records. SQL can do lots of other operations, including optimizing and maintenance of databases. SQL stands for Structured Query language, pronounced as “S-Q-L” or sometimes as “See-Quel”… Relational databases like MySQL Database, Oracle, MS SQL Server, Sybase, etc. use ANSI SQL.

Read more on https://www.guru99.com/introduction-to-database-sql.html

What is the BACnet protocol?

The protocol is supported and maintained by ASHRAE Standing Standard Project Committee 135. Products are certified for compliance and interoperability through BACnet International via the BACnet Testing Laboratories (BTL). BTL Certification is available from several testing facilities, including all WSPLabs, MNB in Germany, and SoftDEL Systems headquartered in Pune, India. Applications BACnet supports most building operations, including HVAC, lighting, fire protection, and physical security (access control, intrusion) devices.

What is Integration in BMS System?

BMS System main purpose for controlling the HVAC system by its own DDC Controller whereas other connected equipment’s in the building can be monitored.

Apart from that other ELV system like Lighting control system, Fire alarm, UPS, Metering, etc can be integrated through over Communication network instead of the hardwiring cable by building network protocols like BACnet, Modbus, Lonworks, OPC and more

Read more on https://methodstatementhq.com/integration-interface-of-bms-system-with-other-services.html

What is HVAC?

Heating, ventilation, and air conditioning (HVAC)[1] is the technology of indoor and vehicular environmental comfort. Its goal is to provide thermal comfort and acceptable indoor air quality. HVAC system design is a subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics and heat transfer. “Refrigeration” is sometimes added to the field’s abbreviation, as HVAC&R or HVACR or “ventilation” is dropped, as in HACR (as in the designation of HACR-rated circuit breakers).

What is AHU?

Air handling units’ condition and distribute air within a building. They take fresh ambient air from outside, clean it, heat it or cool it, maybe humidify it and then force it through some ductwork around to the designed areas within a building. Most units will have an additional duct run to then pull the used dirty air out of the rooms, back to the AHU, where a fan will discharge it back to the atmosphere. Some of this return air might be recirculated back into the fresh air supply to save energy, we’ll have a look at that later in the article. Otherwise, where that isn’t possible, thermal energy can be extracted and fed into the fresh air intake. Again we’ll look at that later in more detail.

Read more on https://theengineeringmindset.com/air-handling-units-explained/

What is FCU?

CU stands for fan coil unit but often engineers will just call them a fan coil, to save time. Fan coil units are very common in all types of buildings, from offices, bars, canteens, even some homes and apartments will have a fan coil unit in them. Fan coil units are used to condition the local air to suit the temperature requirements of the immediate space.

Read more on https://theengineeringmindset.com/fan-coil-units-hvac/

https://youtu.be/MqM-U8bftCI

What is VAV?

Variable air volume is a type of heating, ventilating, and/or air-conditioning system. Unlike constant air volume systems, which supply a constant airflow at a variable temperature, VAV systems vary the airflow at a constant temperature. Wikipedia

What is VFD?

A Variable Frequency Drive (VFD) is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor. Other names for a VFD are variable speed drive, adjustable speed drive, adjustable frequency drive, AC drive, microdrive, and inverter.

Frequency (or hertz) is directly related to the motor’s speed (RPMs). In other words, the faster the frequency, the faster the RPMs go. If an application does not require an electric motor to run at full speed, the VFD can be used to ramp down the frequency and voltage to meet the requirements of the electric motor’s load. As the application’s motor speed requirements change, the VFD can simply turn up or down the motor speed to meet the speed requirement.

https://www.youtube.com/watch?v=g7jFGOn6xfU

What is Chiller system?

Chilled Water System Basics – Chilled water systems in residential HVAC systems are extremely rare. A typical chiller uses the process of refrigeration to chill water in a chiller barrel. This water is pumped through chilled water piping throughout the building where it will pass through a coil. Air is passed over this coil and the heat exchange process takes place. The heat in the air is absorbed into the coils and then into the water. The water is pumped back to the chiller to have the heat removed. It then makes the trip back through the building and the coils all over again.

Read more on https://highperformancehvac.com/chilled-water-system-basics/

What is DDC?

DDC is a controller which use the analogue or digital signals from various devices of a field sensor and actuators and then process and control the system based on the programme written inside the controllers and has the capability to sends the information to another controller or DDC

Read more on https://bms-system.com/what-is-ddc-or-direct-digital-controller-in-bms-system/

What is Ambient Temperature?

Ambient temperature is the air temperature of any object or environment where equipment is stored. The adjective ambient means “relating to the immediate surroundings.” Also sometimes referred to as the ordinary temperature or the baseline temperature, this value is important for system design and thermal analysis.

In a computing context, maintaining an appropriate ambient temperature is crucial to the proper functioning and longevity of computer equipment. In general, a safe range is between 60 and 75 degrees Fahrenheit or 15 and 25 degrees Celsius, although the cooler end of that range is better. Ambient temperatures above those ranges make it difficult for a computer’s cooling system to keep it at a safe operating temperature.

What is Dry Bulb, Wet Bulb, and Dew Point Temperature?

The Dry Bulb, Wet Bulb and Dew Point temperatures are important to determine the state of humid air. The knowledge of only two of these values is enough to determine the state of the moist air – including the content of water vapour and the sensible and latent energy (enthalpy) in the air.

https://www.engineeringtoolbox.com/dry-wet-bulb-dew-point-air-d_682.html

The Dry Bulb temperature usually referred to as “air temperature”, is the air property that is most commonly used. When people refer to the temperature of the air they are normally referring to the dry-bulb temperature.

The Dry Bulb Temperature refers basically to the ambient air temperature. It is called “Dry Bulb” because the air temperature is indicated by a thermometer not affected by the moisture of the air.

The Wet Bulb temperature is the adiabatic saturation temperature.

Wet Bulb temperature can be measured by using a thermometer with the bulb wrapped in wet muslin. The adiabatic evaporation of water from the thermometer bulb and the cooling effect is indicated by a “wet-bulb temperature” lower than the “dry bulb temperature” in the air.

The rate of evaporation from the wet bandage on the bulb, and the temperature difference between the dry bulb and wet bulb, depends on the humidity of the air. The evaporation from the wet muslin is reduced when the air contains more water vapour.

he Dew Point is the temperature where water vapour starts to condense out of the air (the temperature at which air becomes completely saturated). Above this temperature, the moisture stays in the air.

• if the dew-point temperature is close to the dry air temperature – the relative humidity is high
• if the dew point is well below the dry air temperature – the relative humidity is low

If moisture condenses on a cold bottle taken from the refrigerator the dew-point temperature of the air is above the temperature in the refrigerator.

What is Humidity?

Humidity is the presence of water vapour in the atmosphere. The more water evaporates in a given area, the more water vapour rises into the air, and the higher the humidity of that area is. Hot places tend to be more humid than cool places because heat causes water to evaporate faster.

Read more on https://www.goblueox.com/blog/what-is-humidity-and-why-does-it-matter/

What is Relativity Humidity?

Relative humidity (RH) is the ratio between the amount of water vapour (moisture) in the air at a given temperature to the maximum amount of water vapour the air can hold at that temperature. RH is expressed as a percentage (%). A rudimentary way to calculate it is by using a psychrometer to measure the dry and wet bulb temperature.

For example, 50% RH at a warm temperature has more water vapour than 50% RH at a colder temperature

Read more on https://www.idt.com/us/en/blogs/understanding-relative-humidity-and-dew-point

What are temperature and humidity sensors?

Temperature sensors are components that directly translate physical temperature into digital information. Likewise, humidity sensors can measure atmospheric moisture levels and translate that into digital information

check some of the temperature sensors in https://www.tyrrellproducts.com/bms-controls-peripherals-temperature/

humidity sensors https://buildingcontrols.honeywell.com/subcategory/Sensors/Humidity-Sensors

What is damper actuators?

Actuators in HVAC systems control the dampers. By utilizing a low voltage signal, the actuator places the damper at any point between fully open and fully closed. A critical element of an HVAC system, proper actuators ensure that zone dampers function properly – without them, the dampers won’t function at all.

https://www.thomasnet.com/insights/the-significance-of-dampers-and-actuators-in-your-hvac-system/

What is PICV?

Pressure Independent Control Valves (PICV) can help reduce energy costs and increase occupant comfort in heating and cooling coil applications in buildings. A PICV is best described as two valves in one: a standard 2-way control valve and a balancing valve. PICV obtain optimal results because only the necessary amount of hot water (in GPM) and chilled water (in GPM) is delivered to the heating and cooling coils. Standard 2-way control valves allow for overflow and underflow especially if the CV is oversized or undersized. This generates excess water to the pump to compensate for their inaccuracy, which increases pumping cost. The actuators on PICV valves do not cycle as often as standard 2-way valves to compensate for pressure changes in the system that impact the flow, which results in energy savings.

Read more on https://www.neptronic.com/TechTime/20161207/PDFs/TechTime_November_2016_HVAC_Controls.pdf

What is Pre-Commissioning?

• Pre-Commissioning is the activity of ensuring the correctness of the following things
• Cables termination between controller and field side.
• Configuring the Sensors and actuators as per requirement.
• Checking the internal wiring of the BMS DDC Controllers and power it up.

What is the role of Testing and commissioning?

BMS Testing and commissioning Engineer is responsible for followings which are a key role to test from the installation of the system to handover the system to the client 

Installation and termination

• Installing sensors and actuators as product manual given by respective vendors is a vital factor of performance for the operation of the BMS System.
• Identify the best location in the equipment based on which is recommended in the product manual and considering the availability of providers in the site. 
• Based on the sensors and actuators, BMS internal Networking, you need to install right cables from devices to DDC Controllers and terminate it in both sides.

 Pre-commissioning

• Good Pre-commissioning will reduce the most of testing time in the site.
• Pre-Commissioning is the activity of ensuring the correctness of the following things
• Cables termination between controller and field side.
• Configuring the Sensors and actuators as per requirement.
• Checking the internal wiring of the BMS DDC Controllers and power it up.

 Programming Engineer

• Should understand the sequence of operation of various equipment’s in BMS mainly HVAC related like chiller system, Air handling unit, Fan coil unit, Variable air Volume etc.
• Should write the program in Functional block(various method available for different vendors)based on the sequence of operation and given point list.
• Should know to write a flowchart for a given sequence of operation.

Graphics

• Graphics is nothing but to show connected equipment in BMS PC in graphical software provided by respective vendors.
• It needs only the necessary experience of using computer tools.
• Creating equipment’s graphics and adding/respective mapping points to images to animate.

Testing and Commissioning

Testing Equipment

• The program should be downloaded to the DDC Controller and check the sequence of operation.
• Should co-ordinate with respective equipment’s technicians while testing system from BMS DDC Controller.

 Inspection

• Once the system is ready to control and monitor from BMS PC. Then Checklist should be submitted to the contractor/consultant to raise inspection.
• The consultant will check every point from BMS PC to ensure the working of sensors and sequence of operation.

 Training and Handover

• Once testing and inspection finished, we need to handover the system to end-user by providing operational & manual and training.
• Training is the process of teaching end-user how to operate the system.
• How to check alarms, trend log, reports and scheduling.