Intrinsic Safety EX03

Question 1

What is intrinsic safety?

Answer 1

Protection method based upon the restriction of electrical energy. Brings it below the minimum ignition energy require to cause an explosion.
Live is maintenance is permitted

Question 2

What are the three categories of Intrinsic safety?

Answer 2

Exia, Exib and Exic

Question 3

What is simple apparatus?

Answer 3

• A device that is not capable of producing energy to a value greater than 1.5v, 100ma, 25mw.
• Switches, thermocouples, RTD’s, LED’s
• Certification is not required

Question 4

How is certified equipment marked?

Answer 4

It will be marked Exia, ib or ic. If marked in a square bracket [Exia] then it means that the equipment is associated with equipment that it intrinsically safe.

Question 5

What is a Shunt Diode Barrier? (Zenner Barrier)

Answer 5

A current and voltage limiting device, which uses an infallible resistor to limit the current to the hazardous area and a Zener diode to limit the voltage.

Question 6

Rules of Zener Barriers

4

Answer 6

• Zener barrier installations requires a high integrity earth to function
• Maximum of (um) 250v applied to non-hazardous terminals
• Minimum of one 4mm (old standard) or two 15mm conductors required from barrier to earth bar
• Impedance from the barrier earth bar to the star point of the transformer should be less than 1 ohm. <1ohm

Question 7

What is a Galvanic Barrier?

Answer 7

This is a physical isolation between hazardous area and safe area circuits within the barrier.
They give 400v isolation between these circuits
They do not require any earthing arrangements

Question 8

Explain the installation of barriers

Answer 8

• Installed in non-hazardous areas
• Recommended as good practice not to mix Zener barriers and galvanic barriers on the same mounting rail
• Barrier should be mounted so that there is a min of 50mm between safe and hazardous area terminals
• To increase integrity of system, IS and non-IS cables should be kept segregated. Done by looming of the cables or running cables in trunking

Question 9

Junction Boxes Rules

Answer 9

• Do not need to be certified and labelled as simple apparatus
• Cable entry must maintain IP54
• If less than 6mm thread entry than IP washer required
• Box must be labelled

Question 10

What should a junction box be labelled that has IS circuits contained?

Answer 10

WARNING DO NOT OPEN WHEN ENERGISED

OR

WARNING NON-INTRINSICALLY SAFE CIRUCITS PROTECTED BY INTERNAL IP30 COVER

Question 11

IS Terminals

Answer 11

Should maintain creepage and clearance distances.

6mm between bare IS conductors
3mm between an IS conductor and earth

Question 12

What are Ex components and give some examples?

Answer 12

Requires additional cert when incorporated into electrical apparatus or systems intended for use in potentially explosive atmospheres
Ex components are marked with the symbol ‘U’
- A type ‘d’ push button switch
- A type ‘e’ terminal block
- Some cable glands