Water Immersion Pre-Stressing Testing Machine for Composite Insulators

Water immersion pre-stressing testing machine at a glance

• Purpose: checks the tightness of the polymer housing of a composite insulator — whether the silicone rubber housing is still well attached to the FRP core after accelerated ageing in boiling salt water.
• Procedure: the insulator is placed in the cabinet, the salt water is heated to boiling and held there; after the boiling period the unit is cooled and the interface between the core and the housing is examined.
• Acceptance: no detachment and no gap may appear between the core and the housing.
• Position in the test programme: the water immersion pre-stressing test is the pre-treatment of the electrical test sequence — the boiled unit goes on to the power frequency flashover test and the steep-front impulse test.
• Cabinet: 1.2 m3 nominal capacity, 6000 x 450 x 450 mm; temperature up to +100 °C, fluctuation ±1.0 °C, deviation ±2.0 °C.
• Control: digital display, automatic temperature holding and automatic time control.

1. What this machine is used for

The water immersion pre-stressing test — in the workshop it is simply called the boiling test — is the way the tightness of a composite insulator is checked. A composite insulator is a fibre reinforced core with a polymeric housing and sheds moulded or injected onto it. If the bond between the two is weak, water will travel along the interface in service, and the insulator will fail long before its electrical life is over.

The machine puts the whole insulator into a cabinet of salt water, heats the water to boiling and keeps it there for the period the applicable specification requires. Boiling water is a fast way of forcing moisture onto the interface: if the housing is not properly attached to the core, the difference in expansion shows up as blisters, gaps or detachment while the unit is still in the tank.

After the boiling period the water is allowed to cool, the insulator is taken out and the interface is examined. No detachment and no gap between the two surfaces of the material is the acceptance criterion.

2. Standards applied

• IEC 61109 – Composite suspension and tension insulators for overhead lines with a nominal voltage greater than 1000 V – Definitions, test methods and acceptance criteria. The water immersion pre-stressing test is part of the test programme of this standard.
• GB/T 19519-2004 – the Chinese national standard corresponding to IEC 61109:1992, MOD, with the same definitions, test methods and acceptance criteria.
• Specification of the purchaser – where a utility or a project specification gives a longer boiling period or a different salt content, the machine is set to that value; the test parameters are entered from the control panel.

3. Technical parameters

The cabinet length of 6000 mm takes long rod composite insulators whole, so the unit does not have to be cut before the test. Cabinet dimensions other than the nominal size are made to order.

4. Key functions

• Heats the salt water to boiling and holds the temperature automatically for the whole test period.
• Automatic time control — the boiling period is set on the panel and the machine stops when it is over.
• Digital display of the temperature and of the test status, so the whole boiling process can be witnessed without opening the cabinet.
• Temperature held within ±1.0 °C fluctuation and ±2.0 °C deviation, so every unit in the cabinet sees the same condition.
• Cabinet sized for full length composite insulators; custom dimensions available.
• Simple operation — the inspector loads the units, sets the time and reads the result.

5. Typical applications

• Composite insulator manufacturers – sample test of the tightness of the polymer housing as part of the IEC 61109 test programme.
• Pre-treatment before the electrical tests – the boiled unit is taken on to the power frequency flashover test and the steep-front impulse test.
• Material and process development – comparing housing materials, bonding agents and injection parameters.
• Testing laboratories and utilities – incoming inspection and type test verification of purchased composite insulators.

6. Frequently asked questions

1. Why is this test called pre-stressing?

Because the boiling does not by itself pass or fail the insulator design — it stresses the interface between the core and the housing first, so that the electrical tests which follow (power frequency flashover and steep-front impulse) are made on a unit that has already been aged by moisture.

2. How long is the boiling period?

The period is set by the applicable specification. For the water immersion pre-stressing test of IEC 61109 / GB/T 19519 the period is 42 h; where a purchaser or an older specification requires a longer period, the machine is set to that value, because the time control is free to set.

3. Is salt added to the water?

Yes. The test is carried out in water with salt added, which makes the ageing more severe than plain water and shows up a weak interface faster. The salt content follows the applicable specification.

4. What is checked after the boiling?

The insulator is taken out when the water has cooled and the interface between the FRP core and the polymeric housing is examined. No detachment and no gap between the two surfaces of the material is the acceptance criterion, and the unit then goes on to the electrical test sequence.

5. Can a full length insulator be boiled, or does it have to be cut?

A full length unit is boiled. The nominal cabinet is 6000 x 450 x 450 mm, long enough for long rod composite insulators, and other dimensions are made to order.

6. Can the machine be used for the water diffusion test as well?

No. The water diffusion test is a different test on a short sample of the core, with the voltage applied afterwards; it needs its own test device. This machine does the boiling pre-stressing of the complete insulator only.

Related composite insulator test equipment

In the IEC 61109 test programme the boiling machine is used with the water diffusion test device for the FRP core, the tracking and erosion test machine and the power frequency test equipment. The complete range is listed under insulator electrical test equipment.